He is 20. And living at home.
For a start? Your house, your rules. PERIOD.
If he is going to live there, you need to protect yourself. Even if you kick him out again, you need to protect yourself - he may try to break in.
I'd be pulling internet access. Turn it off when you go to bed. Move where the router is - to a locked room - so he can't manually turn it back on and/or connect his xbox or computer to it. Change the Wi-Fi password, and don't give it to him. The computer isn't helping him.
Lock up all valuables, and all knives. Change account numbers / card numbers if you have to - he might have them written down somewhere. If mail comes to your house, switch to a PO box, so he can't steal the mail and use the account statements to get the credit card and bank account numbers.
Get rid of all guns, or find an off-site secure storage system. You don't need him having access to a gun under any circumstances.
Please! Don't be so hard on yourself. Because in reality, you did NOT cause this. Sure, you might have made some different parenting choices along the way. MAYBE those changes might have made a bit of difference... or maybe, not. We do not generally CAUSE mental health problems. Sure, growing up being severely neglected or abused can cause mental health issues - doesn't sound like that was the case at all. Your son has a HEALTH issue. He also is still capable of choices. Stealing is a choice, for a minimum.
We can only go forward from where we are. Warn him: all thefts will be reported to police and prosecuted. Then, protect both of you as much as possible. If he threatens suicide, call 911. If he threatens you, call police. He doesn't get a key to the house - he can only be home during certain hours, so that you are free to come and go and do what you need to do.
I have a young adult son with multiple challenges including mental illness - and he holds down a full-time job, and generally abides by the (few, simple) rules of the house. He chooses to do that. He has made the decision that it is better to have some money than to not have money - at least he can do a little of what he wants to do. The stability of employment doesn't hurt his mental health at all (stability is a good thing, so is getting out of bed every morning). He still has major issues, and is still difficult to live with. I'm telling you this just so you can see that these kids do have options and can make choices.
