Pausing for a second to make sure it's actually a deer, for starters, doesn't run the risk of literally driving you insane.
Secondly, even the Sight is fallible. It is subject to interpretation. There are several instances in the novels where Harry, even using his Sight, just doesn't understand what he's looking at.
Thirdly, the entire basis for the gaining of a Lawbreaker stunt, and the represented 'corruption' is that somewhere, deep down inside, you believe that what you did was right. For the First Law, that you believe using (perhaps even potentially) lethal magic against human targets is (even sometimes) the right thing to do.
But it's well established, in both the novels and the rpg books, that using the same kinds of magic against non-humans is A-OK from the perspective of the Laws.
And since the Laws reflect belief, and killing non-humans is indisputably not covered, it must follow that believing you're killing non-humans is similarly not covered.