Because there seems to be some confusion:
i.imgur.com/j2wUStB.jpgThis spirit possessing Mario's clone is the <<
www.pikminwiki.com/Smoky_Progg|Smoky Progg>>, a boss from Pikmin. It seems to be a normal spirit; rather it's a basic <<
i.imgur.com/oScfsTa.jpg|Novice rank spirit>>. As far as I can tell, there are no generic "evil spirits" original to Smash Bros. like the Subspace Emissary's shadow bugs. All spirits originate from another series. Here we can see that it's these normal spirits that are causing the red eyed possessed look.
i.imgur.com/EZdqegn.jpgNote that even Shantae, an Ace level spirit and a hero in her series, causes the red eyes. (And grants an automatic Killing Edge that deals extra damage, deer god watch out.)
This is where we come to the difference between the Spirits mode and the World of Light. These are two different modes, and the mechanics work slightly differently. Spirits mode is the multiplayer, where spirits are "lending their power" when equipped and are not necessarily possessing fighters. (Unless it's a DBZA Piccolo-Nail-Kami situation) Meanwhile, the World of Light is the new story mode, and part of the story is that characters that were captured are being cloned. The clones are soulless shells, which the spirits then possess and control.
So in World of Light, the story is that the clones are being completely controlled by a single spirit inhabiting them. Once you defeat and acquire the spirit, however, it becomes equip-able and 'lends its power' instead of possessing the character it's equipped to. The ones you fight are possessors. The ones you equip are not.
Makes sense?
