Having a living thing killed because it did something you don't like, that it doesn't understand is wrong, is so incredibly heartless. Pets aren't toys meant to be tossed when you get tired of them, they're living creatures deserving of a good life.

Now if it was peeing everywhere because of a terminal medical issue causing it pain, that's a bit of a different story. I don't know the situation he and the cat were in. But if it is just as the parent comment says, why kill the thing instead of giving it up for a chance at a better life?