What possessed my brother to do this?

Please keep this anon for obvious reasons.

I was called last night at 3am after I enjoyed a fun night out at uni by my sister. She said my 15 year old brother was arrested on assumption of armed robbery and offence. And today there were witnesses who said he did it and charged him.

The weird thing is, my brother and I always had a good relationship, I always adviced him about girls, told him to stay off drugs and people who will influence him badly. And I wasn't like a mother/father person to him, he knew he could tell me things he couldn't tell our parents. Now I'm just wondering what on earth possessed him to rob someone and then put a knife to someones neck. His courtcase is on monday, and although I'm hoping he won't get a harsh sentence, I'm so infuriated right now that I hope he gets what he deserves for doing those things. But theres still someonething in the back of my head saying he didn't do these things...

Does he smoke cannabis?

Edit: I'm not taking the piss btw. I work for a mental health NHS trust, and have met people who have done what your brother did whilst being high from weed (and other drugs, but mainly cannabis).

call him and talk to him.