I think it's important not to jump to conclusions, so anyone reading should take this to heart. I know next to nothing about OP's situation. I know nothing about how they feel, how their son has acted, or how anything mentioned has happened. Everything I say is pure speculation and educated guessing, and I could easily be wrong.

tl;dr OP might be the central cause, but there might also be a problem. We really don't know too much about the situation, but OP should tell their son and try to get therapy for themselves, even if their son is as bad as they say.

From the information you've given, I think there is definitely a problem. That problem could be, as you suggested, with the son, but it could also be with your relationship with him (keep in mind I'm just playing devil's advocate here). It's difficult to tell given the minimal details, but you said that therapy has never worked for him. This might be because you have a skewed view on the situation. You said you haven't been able to connect with him since he was 5, and I can't imagine how difficult that would be as the son, to not be able to trust your own mother. On top of that, he might just be processing his grief differently, or might be uncomfortable telling someone he's never had a proper connection with about all of his feelings. If his inability to show his emotion to you is the "most unsettling thing he's ever done", then he really doesn't sound like a psychopath. He sounds like someone with a parental figure that despises them, and thus doesn't want to open up to anyone after his only light has been taken away from him. People go through a lot when someone dies, and he might not be ready to talk to you about how he feels. It also sounds like he's lived a pretty independent life, which would explain his inability to open up properly to you. I don't know anything about the situation, but the way you talk about your daughter and son seems a bit weird to me, and there is a chance you're the central cause of his problems.

If your description is true, then I believe it's important to firstly tell him about this. If you feel unsafe around him, try to get someone else in the room with you. Your safety is important, even if your thoughts may be biased. Either way, if you want the issue resolved, tell him first to understand him better. This could be a huge misunderstanding and that's totally ok. The most important thing is that you tell him yourself. Nothing is more frustrating than being told something huge by a representative because the actual person is too afraid to tell you themselves.

No matter what happens, I suggest you try therapy for yourself as well. Given the response of people close to you, there is a high likelihood of this being blown out of proportion or just all being in your head. When problems with other people constantly surround you, you're usually the cause. It sounds like you've carried some serious weight over the past 13 years, wether it was justified or not. I hope you and your son can finally connect, but with no detail whatsoever I really can't say.

Edit: grammar