Is there any value in drawing lines between a mental disorder, and any other mental trait? And similarly, is there any value in judging someone as good or evil?

I think the answer to both questions is an obvious "no".

Ultimately, all that matters is what we can predict a person's behavior will be. If a sadist kills someone for no apparent reason other than the thrill of it, then we assume they may do it again, and so we lock them up. Similarly, we all know very well what a Trump presidency looks like. And if we don't want a repeat, we should not reelect him. And we should continue to press charges against him for any other crimes he commits. Simple as that.

People love accusing other people of being evil, because it makes them feel better about themselves. But it's ultimately an entirely pointless - and therefore decidedly petty, vain, and immature - exercise.