Trigger warning: animal abuse, suicidal idealisation, physical abuse, verbal abuse, disturbing, pedo behavior, 

I'm really scared for my little brother. I'm worried he may have something deeply wrong with him and there's nothing I can do that will help fix it and I just don't know how to exactly help him. 

He's done a lot of fucked up shit in the past when I lived with my parents. Such as fucking my cat and covering it with semen(I cleaned it up after coming home to that and refused to let him touch my cat afterwards), waiting until I fell asleep to sneak a peek at my body several times, stealing money and having to get a talking to to get any of it back, getting physical when angry, refusing to talk when I try and talk to him about something that pissed him off, talking to and dating 10-12 year olds, refusing to go outside or talk to family, gets overwhelmed with loud noises or crowds of people(this isn't anything concerning but I figured I might mention it anyway), peeing in a cat litter box twice because he was too lazy to go to the bathroom(who even does this?!),  sneaking and staying up late to play video games all night long or watch porn, snuck on our dad's phone to watch child porn(weird.. But okay), played with fire around a dumpster and in the house resulting in the landlord getting a few calls, doesn't seem to have a good grasp on how to have a normal conversation without redirecting it to something he wants to talk about only, unhealthy dependency on parents and refusing to get a job or earn anything for himself aside through social media platforms, refusing pictures so he can "remain mysterious", told me he had knives hidden all around his room and was cutting himself, had a noose made ready to commit. 

That's just the past few years. He's 18 now and this started when he was maybe about 13-14.

Last time I talked to him, he told me he had concerning detailed thoughts about murdering certain people who did him wrong and including the family dog. He told me this and that he doesn't want to talk to people he doesn't know, only people he trusts(basically immediate family). He's still talking to 10-12 year olds, and is becoming very popular on tictoc. 

I feel at a loss to help him and I'm terrified he might be a psychopath or something to that degree. I've read too many times that signs often show up in early childhood, and I just don't have the knowledge to handle this sort of problem or give him any helpful advice other than to talk to a professional. 

What am I supposed to do? How do I get him the proper help? How do I help him?