Killing Skeeter is about the only truly questionable action of Quirrellmort that I can remember.

Even here, I find it hard to hold it against Quirrell. Rita made a career of libeling others, blithely unconcerned about the harm she caused to their lives. In fact, she seemed rather smug and self satisfied about exercising that power. Quirrell even confronted her and asked her to stop. She had a chance and chose not to take it. She was destroyed in the act of her preferred crime by the person she intended to harm.

I suppose I have a bit of Quirrell in me. He takes a grim satisfaction in the poetry of citizens being destroyed in the same prisons they demanded be built. The word for that is justice. A harsher justice than I'd want to seen meted out, but justice nevertheless. I wouldn't have squashed Skeeter, but I can't condemn Quirrell for it either.

And yes, Skeeter likely had children who would miss her. Just as good people have some bad, bad people have some good. Recognizing that the world is not black and white shouldn't stop you from seeing that some grays really are darker than others.