I drowned my aunt's cat in her sink
What happened to make you drown the cat?
The cat liked to piss in the sink, which annoyed me. Also it ate my dog's apple cores.
Even if the cat behaved in a way that annoyed you, killing it is an extreme reaction. A cat is an animal and does not have self control, while you do. You should use more wisdom and proper judgement in situations where you are annoyed, and not have such extreme reactions.
It will be fine, I'll replace the cat.
It is not even a little bit okay to kill a pet because it was bothering you. Replacing the cat is not really a solution. You could end up in trouble for animal cruelty if your aunt turns you in.