Is he/it outright Evil though?
Armus was was abandoned on that Planet. He was acting out because of that.
It's hard not to feel a teensy bit sorry for Armus in his final screams as he realises that Picard has tricked him and he is going to be abandoned. Picard's final line though, is awesome.
"I'm not taking you anywhere"
On a fundamental level I actually find her sacrifice in "Yesterday's Enterprise" somewhat trite. It's what we expect fictional characters to do when they face death, it's caught up in the mythology and romance of sacrificing your life to save others, noteable and honorable things.
I've quoted this bit, but your whole post hits the nail squarely on the head IMO.
What's interesting about the "predictable deeds of action heroes" angle is that it mirrors the "fairy tale" angle related about Alt-Yar's eventual fate. To paraphrase, a Romulan officer saw her and fell in love at first sight, saving her and her comrades from death at the hands of the Evil Establishment. Clearly by the mid-to-late season of TNG, more convential storytelling tropes had taken hold.