Well, in terms of how 'evil' Mad Max villains are I'm going to have to put the Toecutter above everyone else.
Here's why:
All of the Mad Max villains except the Toecutter show some kind of empathy and in their head they think they're doing the right thing. The problem is - they're doing it in a completely wrong way. Immortan Joe thinks he's helping but he's actually a tyrant. Auntie Entity thinks she rebuilt civilization but in fact she's a tyrant. Humungus clearly understands that 'we all lost someone we love' so there's empathy in him, but 'he does it his way', so he's killing everyone who doesn't abide.

Now Toecutter:
Toecutter realized that in this crazy world that's spiriling out of control he can do whatever he wants. So took a whole bunch of deranged individuals under his wing, the crazier the better. Because he could manipulate them into whatever he says. The Nightrider was clinically insane and was 2nd in command after Toecutter. Johnny The Boy was crazy insane, so he was chosen by Toecutter as his future successor. Then you have Zinnetti who's a calculated psychopath. Clunk who was mentally a child willing to kill at Toecutter's notice. Cundalini and the rest of those crazy folks. In other words - Toecutter used crazy and mentally disabled people just to wreak havoc. Because he could.
There was no other reason. The dystopian world of Mad Max was his playground. There was no ambiguity about his actions at all. That's why he's the worst of the villains in all of the Mad Max movies.