Well, really now, Harry eats meat. In other words he will accept the death of a nonsentient creature for purposes, not even of keeping himself alive for a few additional hours - he could eat veggies - but just because they taste good. To preserve life? Pff. The argument about sentience is one he ought to have, but conditional on unicorns being nonsentient, they are just not valuable compared to additional months of life.

His best friend just died and it looks like his mentor is next. Good recipe for motivated reasoning.

There aren't any clues that unicorns are sentient, so there's no reason why Harry should find QM killing a unicorn more thought provoking than eating pork.