NTA (on a technicality)

If you cut out all the context and just posed the question as “someone said some sassy shit which made me laugh, does laughing in private make me an asshole?”, then I’m sure you’d get a lot of replies saying something like “you can’t choose what you find funny”

However, YTA for other reasons:

• letting the bully say ‘inbred freak’, ‘piece of shit’ and ‘kill yourself’ is not “sassy”. Being sassy is saying something like…
• labeling someone as pathetic for having a mental illness is judgemental and hypocritical.
• by failing to call out the bully’s behaviour, you endorsed it.
• if you laughed publicly, then you encouraged it.

Yes

YTA big time, the kid has mental health issues and was being bullied and as his “friend”, especially an older one, you should’ve done something to help.

With friends like this, you don’t need enemies.

encouraging that if he was so mentally ill, he SHOULD kill himself. I found Bully rather sassy and the things he said were funny

I’m amazed that you were able to type this out and still were not sure if you’re an asshole… YTA just in case you’re still wondering

Glad you now free from that phase