OP I went through this with my DS.
He wasn't violent though.
Is he in school or working at all?
As hard as it is, when kicks off, you have to call the police on him. He needs a rude awakening.
As for the trust part, it's good your DS doesn't know about it. At 18, it's very unlikely he pays attention to mail.
I would keep the letters from him. Of course he will waste it on drugs.
You may be in more danger from him when he goes on a bender. He is also in danger having the money and a lot more drugs, alcohol, etc.
My DS did know about the money and never in a million years did I dream he would have had a drinking habit. I didn't have any say about the money and there was nothing I could do about it.
The way it was set up, it was certain amounts at 18, 19, 20 then 30 and 35. Well he found out a way to sell the 30 and 35 year parts and he did. It's so disappointing that he wasted all the money and it was a lot.
Do what you need to to protect yourself. Is his father in the picture?