Well, the magical world is full of sentient or half-sentient things, from house elves to phoenix. The hypothesis that unicorns are half-sentient like a phoenix can't be excluded a priori, it's something a rationalist should inquire before taking any decision. And the scope and nature of side-effects should be inquired. Harry doesn't even do the simplest inquiry, asking Quirrel about it, but jumps to conclusion with incomplete data, doesn't sound like him at all.