So I'm in a big pickle.

My uncle has an apartment but still stays the majority of the time at my grandparents house. He used to hate that my mom, brother and I lived there and now he is doing the same. He also hates my brother who is autistic since he used to live with my grandparents too. Now he's freeloading off my grandparents and he's very threatening to anyone who tries to get him out.

Now he threatened to make my mom's life a living hell if "she doesn't stop butting in" Then the thing is, if he doesn't leave - my grandparents could get in a lot of legal trouble so we've been trying to help them for a long time now.

I'm stressed and tired of all this fighting that I've been dealing for a long time now and honestly, I'm freaking tired of it.

My uncle keeps getting away with everything and I'm pissed

Why would your grandparents get in legal trouble? The only solution I'm coming up with is telling your Uncle he's an adult and has an apartment (He does have a job, yeah?) so he has no reason to stay with his grandparents.

And changing the locks while putting all his stuff outside when he's gone from the place, since he's not there all the time.

They have housing assistance and if someone lives there without the housing knowing can get them in a lot of trouble. Like 5 years in prison plus 10k fine

Oh, damn. Yeah, I'm not sure what else to do other than kick him out. How does he stay there without the assistance knowing? Maybe inform them that he's not allowed in there, and have them remove him from the premises anytime he shows up? I know that sounds harsh, but from your description, he really needs a kick in the rear.