My in-law is acting weird so I want to call him a gigantic fucking baby.
Wow. Do you really think he is acting that immature?
He throws a lot of tantrums and cries a lot, he's all the time ruining everything, he won't get over his sadness
Its better to be straight to the point when dealing with family. Any type of passive aggressiveness will just get bottled up and end up making matters worse. Although you came across very blunt and rude, that is even better than not saying anything at all.
I'm just one of those people that would kill himself before hurting someone else's feelings.
Yeah, well you may need to be aggressive and firm. You don't have to try hurting his feelings but a little confrontation when trying to help someone is good and can help him. You just shouldn't give him passive aggression, as that doesn't help anyone.