Fi can be used as a weapon of the finest caliber if wielded 'correctly', so an INFP villain is certainly possible, given the right motivation.
That is true for any type - hit close enough to the very core of their survival (psychologically or physically), and make the threat real, and present.
Then watch them go nuts.
For TJ's it's often the feeling of unpredictability and loss of control that makes them go batshit and overcompensate by taking over the world in order to organize it properly
For TPs it's often a sense of wanting to watch the world burn around them for what it did to them, and seeing everyone else suffer for once (inferior Fe), or having their experiments and life's work taken from them or severly limited (NeTi)
For FPs it would be watching the world decay into a place where you're so surrounded by pain and misery, you cannot take it anymore and try to right all the wrongs you see - by being wrong yourself as its the only way to cut through the mess.
For most FPs, it can also just be more upclose and personal (you're inflicting damage on others in a way that I was damaged by those I trusted and couldnt stop, therefore I will annihilate you in self-defense and the defense of others - aka empathy gone bad through projection onto the person you relate most to)
In that regard, INFPs, with their Fi-dom and their inferior Te could very much get into an 'must teach the world right from wrong and make them adhere to my standard' rampage. Iow, follow the moral code or get eliminated (because there apparently is no saving you and you're poluting the universal pool of energy).
And the weapon of choice can either be something to cause actual death, but most likely, for INFPs, if they have mastered their Fi to the fullest, I'd say they just skin you alive emotionally, layer after layer, rubbing salt on the wounds as they go, til you're a quivering mess on the floor. And then, the rebuilding process can begin, to put you back together - if you are worth , salvaging, in their eyes.
Don't underestimate the desire to bring forth that idyllic world they crave in Fi-users - and to do the dirty work to get there if they're driven enough to despair and grew up in an environment that polarized their moral compass to be exceedingly black and white.
The rage and frustration that is created due to reality not matching the picture in their head of 'the way it should be because otherwise it feels wrong and hurts like hell' is like a volcano inside - especially if that reality gets progressively worse or 'fights back' in some way, when you attempt to affect it. At some point, something just clicks and goes 'alright, if you wanna fight dirty....let's!', vigilante style. The temptation to go there is pretty big considering the world we live in, at times
Even worse is Fi-users that have given up all hope of working towards that idyllic picture. The pain and rage from that just makes them bitter and often sadistic in lashing out at others, who they blame for 'making them grow up and face fact'. And hell, if everyone is fuckign up the world anyone, why not just join them. Apparently, it's the thing to do.
