Does anyone use cowplants in mean ways? Evil sims, black widow, ect.

No. As others have stated, the cowplants are much less effective in this iteration. An AI controlled sim rarely goes for the cake a second time, or at least before the moodlet wears off. When I feel the need to end a sim I use alternate methods such as Burst of Anger from Vampires or Marked for Death from Jungle Adventure.