Dogs don't really even understand *what* you're saying, even when you say it out loud. It's *how* you say it.
I could say in a really silly, happy, high-pitched voice while jumping around like a kid at Christmas, "I don't like you, doggy! I'm going to give you a bath! Ten baths! And then I'm going to put you in a room with 20 vacuum cleaners that are turned on at once! And THEN I'm going to take you to the vet, and have you put to sleep!" and the dog would be wagging his tail and really happy and excited because he didn't understand a word I said, he'd only assume I was happy and pleased with him because I was acting like I needed to switch to decaf while talking like Minnie Mouse.
The thing is, even if I was in a really horrible mood, those would probably be the nastiest things I would think of doing, and it would be pretty tame compared to ripping its tail off, stomping on it and kicking its brains out.