It sucks that you got in a fight and it does sound like the guy is crazy and/or on drugs but...
scubamage wrote:
the girl wouldn't let us call the police because the **** was her boyfriend
Your duty here is to report this man to the police
immediately, not to keep his girlfriend happy.
scubamage wrote:
He started trying to pick another fight, and followed them almost to our doorstep where I was standing with a knife
This is probably not the best idea. You have already demonstrated that you and 3 other guys are not able to restrain this man. If had had wanted, there is a good chance this weapon would have been his. He is the crazed attacker here and you don't know what all he is capable of doing. If you are going to pull a knife, you need to be trained and prepared to use it to kill or severely disable him if he decides to proceed with his attack. I doubt this is what you intended. It sounds like you were just trying to scare him away and you got lucky and it worked. The smarter thing to do would have been to lock yourself inside and call the police.
I understand that he had done some extremely violent things to you guys, but there was a point where what he had done was over, everyone was ok, and he had left. This would have been a perfect time to go inside, lock the doors, call the police.
I'm not trying to pick on you because you were definitely in a bad situation and it is hard to know how you will react when put in that scenario. But, I do think you could have made much better decisions here.