He killed a cat. Talking cat, but still a cat. Yes it wasn't nice but in response Sting killed a man. And not an ordinary man - a leader of strongest guild in Fiore - so he was somehow influential. Killed in a town full of Rune Knights - a magic police force. No, I don't think he will be able to escape it in any legal way.Assuming the bastard is actually dead I doubt he would actually be convicted of anything for lashing out at someone who just murdered his friend right in front of him, especially if this is a feudal society.