Should I cut ties with my friend over this?

My friend paul can be quite mean sometimes and we tend to clash quite a lot because I stand up to him when he's in bad modd taking anger out on me and others.

He started hanging out with a naive guy from around the block named John. 

John doesn't have any other friends and lacks street smart and gets bullied by Paul and some of paul's friends.

When john calls into Pauls house, paul doesn't answer the door and makes john climb through the window.

Last week I got annoyed and got up and told John not to climb through the window and that id get the door for him. Paul wasn't happy and when paul was mocking john an hour later, I confronted Paul and we had an argument and Paul threw me out of his house.

Paul apologised the next day but I've been ignoring him and considering ending the friendship, with bug reason of how he's treated john, an innocent victim.

Should I end friendship and what do you think of people like paul picking on someone like john?