I can think of a viable RP angle for an evil healer. the "doctor" with a god-complex. Who gets off on having the power of whether people live or die at his whim.

There's the guy who's just a natural combat medic.

There's the healer who has a hero-complex and is a glory hound who is not a healer for selfless reasons but because he likes the attention.

I could keep going, it's silly to try and cram any one class into a box of "if you play this you must act this way". I think the closest you can come to that is paladin's with the code and that is a fundamental part of what a paladin is. So even though it is somewhat of an exception, it still isn't, because there are so many different flavors of paladin.

As to plague, it isn't evil. That's like saying "Swords are evil, their only purpose is to shed blood". It's a tool, and if it comes from a goodie-shaman it is coming from the righteous wrath of a goodie imm channeled through their priest.

I don't really know what the answer is for revamping shamans (god knows they need it), but I don't think this is it. I applaud your effort and the fact that you are at least putting out an idea, even if I don't think it would work well.