if you wanna make a healer that is evil just make something like a vampiric zenyatta, with some sort of buff they put on people to give them livesteal. or maybe a kinda tech-necromancer that molds hard light into shields and minions of some sort. there's plenty of ways to do it^^
EDIT: actually thinking about it, katya volskaya is making deals with the omnics, maybe we are gonna get an evil omnic hero? that would be kinda cool.
Blizzard seems to have decided that every character must have a gun that works in a different way, and healing too. Its hard to keep coming up with different healing mechanisms apparently.
Your idea reminds me of the mad milk from TF2, I wouldn't mind that. My only complaint is that a lifesteal healer could never be anything other than an off healer, so if we get that we should get another primary healer. Else comp players will get mad at support mains for not picking Ana or Mercy (lord knows it already happens).
I think the original intent was that Symmetra would be an evil healer. She works for a villainous faction yet is well intentioned, and her kit used to include direct targeted "healing". Of course as we all know, Symmetra would be better categorized as a defense hero. I remember back on release, in quickplay people would get mad if you had Mercy, Lucio, and Sym all on the same team because the warning would come up that there was "too many supports."