I sent that guy a message containing a formal excuse for my rude behavior.
He hasen't responded yet.
I sent it via steam, he hasen't added me yet, but does that matter?
I hope it came through to him, and that he understood what I had to say.
I'm still scared. What if he still wants to beat me close to death with some gang?

If he threatens you in the message, then you save it, copy it, and take it your brother/parents/the police.

I just hope that I get a warning.
What if he'll just send some guys to beat me up?
What I'm not aware of his actions before it's too late?

You sound like you're having a panic attack. At this point, since you're having a hard time calming down, you need to tell your brother and parents that you're scared. Period. They can help you far more than we can. Ask if you can have pepper spray or a taser. Have friends stay near you when you go out for a little while. But mostly, if you're scared, then you NEED to go to someone. NEED TO. NOW. Seriously.

I'm thinking of a therpaist.
I've been planning to go to one before, because of depression and anxiety attacks, so that might be a good option.
I'll try to contact him again, if that fails I'll go with your protocol.