This is just a sort of topic that's been on my mind for the longest time. I personally have care almost exclusively for the psychological aspects of the fetish with next to no real need for anything else. I find I have a deep respect and appreciation for this on both platonic and non-platonic levels and that new concepts introduced in association to this concept can enrich my appreciation for it on both levels.
Even as type this I do have the feeling that many won't understand but that's what I hope to attempt to change a little with this forum topic.

My main question is about the inherent sadism, dormant bitterness and cynicism that can be coupled with this concept quite regularly. On this site I find that it's not that bad (lately) but in other content in can be quite heavy and even off-putting. I understand that even if sex and other erotic matters aren't involved it can still be quite dark when a physical reaction is invoked but does this make the individual altogether evil?

Now, don't get me wrong. I know it's all fantasy and has no baring on reality no matter how realistically portrayed. Therefore it's not I'm judging anyone else or even myself. I feel like the slavery fetish and the H&MC fetish do not automatically overlap. This is more a matter of purity of concept than moral principle.

I know vanilla H&MC content is somewhat of a stretch but at the very least, might we call it a "swirl"?

Sadism is not inherent in the mind control fetish. I know plenty of people who enjoy hypnosis and mind control, but do not enjoy sadistic things at all.

As for why some level of sadism, bitterness, and cynicism does show up a lot, it probably has to do with just the dramatic principal that people tend to enjoy conflict in a story. The Dom being somewhat sadistic can lead to a lot of interactions between the sub and Dom to provide dramatic conflict while the sub falls under the Dom's control; or to demonstrate how much control the Dom has over the sub.

I really haven't written many sadistic Doms in my work, actually. I don't really avoid dark themes; but I guess I just happen to naturally lean towards happy trances and comedy when I'm making things.

Mind control and Hypnosis don't inherently have anything to do with sadism or slavery. The core of the fetish is a power thing; either having power over someone else or being powerless. Some also have the alternate view that it can just be about the emptyness or calmness that comes with hypnosis and trance. Some dominants tend to just get off on watching others in that state as well.

However the sadism happens because there's also the power aspect to the fetish. You get off on making people do things or being made to do things. The power disparity is magnified when you're making someone else do/being made to do something you don't want to do. That means that some people - yes, both on the dom and sub sides - get off to the situation more the more degrading or unwilling it seems. It's not necessary, it's a magnifier. Like pepper in your food it adds spice to the right people. What's tasty to one person might be overwhelming or repulsive to another, or else bland and flavorless.

There's a spectrum in the community. Some people only like fluffy light and happy with nothing overtly sexual, some people like a bit of happy, light power play, some people like heavier power play and some people like very extreme stuff.

There's nothing wrong with any of those perspectives and desires, including your own. There's an appeal to all of it, and it's not wrong to dislike any given perspective. It isn't a package deal. Some people want a light trance, some people want to be treated like cattle and be bred and degraded. As long as you can respect the perspectives and limits of other people everyone is free to enjoy what they like.

Wonderful answers already provided by both Changer and cypress, I feel, but I guess this is as good a time as any to throw in a thought I've had for some time now:

A hypno/MC fetish is essentially an offshoot of BDSM.

Discuss away.

I can see that easily. If we had a name for it, I'd call it Mental Bondage.

It is an isn't. It is in the power play aspects, but isn't for the people who just like the empty trancing.

Hear that quite often, am the living proof to the contrary. Love hypnotic/mind control, don't care even a bit for BDSM (and I would fight like my life depended on it if anyone was to force me into doing that kind of stuff ^^). I'm pretty sure I'm not alone with this.

You can't just associate hypno with other fetishes with a power play component and claim that it is a offshoot of them (like BDSM or rape fantasies [although I am not by any means putting those two things on the same level]) because there is even dom/sub play in vanilla sex (handcuffs anyone xD?) ;P.

Interesting responses thus far, thank you all very much, btw. I also think that you're a little wrong to say that H&MC is an offshoot of BDSM. Rather I think it's more or less compatible with pretty much all fetishes.

(The Mysterious King of All Fetishes, if you will)

This is also something I have been thinking about as well to be honest, on top of other things and also how people in general interpret mc/hypnosis things and why.

But I think the way cypress said it pretty much sums up how I feel about most things and also why it's important to know what your limits are in that aspect.

I guess it's really about how people interpret the whole power play aspect and stuff they are into I suppose.

It is the one fetish... TO RULE THEM ALL!!!

There's a certain connection to it, in that both communities have "dom" and "sub" sides, and the two can overlap (as any fetish can), but not necessarily. We have plenty of S&M'y images here, but plenty that are not.

Sadism, ownership, treating another person whichever way you please, owning their body, owning their mind, -> MC-dom-fetish. The logic is there, but this is still only true for a subset of the community.

I tend to think of it as a different kind of arousal than what's most frequently considered. Much like getting an erection, getting a trance puts you into a vulnerable state where someone else could easily take advantage of you. Also like an erection, the tranced state feels very, very good (if done right). Now, just because I like having an erection does not mean I'm into everything another person could possibly do to an erection. There's a lot of bad stuff someone can do when that thing is sticking out, but a lot of good as well. Same for trancing, just because I like it does not mean I'm into every imaginable scenario involving it.

I hope this summary has been helpful in some way.

Can't write a short response...
The things that get me are the emphasis on the dominating force, a feeling that I can only ascribe to as a emphasis on Thanatos (death-fantasy) on both the dom and the subs side, and the extremes the fetish seems to develop to rather quickly. This is on the fetish writing side of things, but some of it still comes through in Hentai.

I would say these are distinguishing elements of MC, which sets it apart from BDSM (there seems to be a lot more push and pull between the dom and sub in bdsm). Hypnosis gets caught up in the mix because it is an altered state of consciousness.

I'd say three different things, but related.

So I guess I'll use this opportunity to talk about dojin.

A lot of MC content, especially Japanese dojin, is actually just an excuse to get some people fucking without any concern whatsoever that the mind control itself is a fetish. (In fact, not being a particular fan of rule 34, I make sure to switch "dojin" search off at exhentai because I'm not into anime and I know almost all of them will not even have enough explanation or characterization for me to enjoy watching two random people fuck, which frankly is a lack of characterization so impressive that I would otherwise think it impossible.) Those are huge snooze-fests for me because they spend one page saying "ok now you'll do whatever I want" and the rest of the book is just them fucking with no indication that there was any mind control involved.

For these works, there's a certain level of sadism from two perspectives: First, it trivializes the submission of a human being into something not even worth getting aroused over, and really puts a spotlight on the similarity between MC and straight-up rape/date-rape. Second, subs are very frequently allowed to protest because of the surprisingly-pervasive meme of Japanese sexuality that a "pure" girl who is not entirely willing is far more desirable than one who is all-in. Their media falls into "ech kinda rapey" on an extremely regular basis, and when MC is used as an excuse to get to that point, it feels like a sadistic use of MC.

not all types of Mc are equal

take bimboization, for example; one of the reasons I like it is it puts the subject into a state where they willingly participate in sexual situations - depending on the bimboization, the controller might actually have little to no control of the subject after the initial transformation
.... of course, there are other versions were it's a form of humiliation, which feeds back into the BDSM and slavery aspects. Of course, often the subject is so addled by sex they become dependent upon the controller, which again goes back into the slavery aspect

further, any modular mental change, such as increased lust, ignoring things they see and hear, or slow changes in beliefs that build upon another - might all lead to situations where one is effectively enslaved.... but also not really

what of possession? The mind is controlled, but so directly that the body is also the controllers - rarely is there any form of slavery present in that condition, although the person might be effectively voided

also, other forms of corruption and/or transformation; into an atavistic being, or jokerization, or a creature with it's own inclined instincts

and, has been noted by others, there are examples of hypnosis where the participants are willing, and where the Mc can improve or empower the subject