Someone asking for favours

A mate at the time asked for favours quite frequently. I shouted at them and swore. I apologised but to no avail. Tit for tat was texted. They called the cops,so that was that. I feel like a monster. That was nearly 2 years ago. Since being on an assertive course i sit down and discuss things instead of flipping my lid. How do i get past this hate that i feel inside of me please?

I tried setting boundaries. I now know that I wasn't clear or firm enough.They threatened to break friends with me a few months previous for not answering the phone,the majority of the time i would.They came round and apologised. I accepted. Perhaps i was expecting a double standard? They would also talk about their problems alot and turn up unannounced at teatime. I should've sat down with them and discussed the problems. They also made snide remarks to me,like "You're not as ill as me." "You don't understand." The other mate who was also my mate,i broke friends with a few months prior to the police being called on me. They too kept asking for favours and talking about their problems,would want me to eat when they eat despite my routine around food. I said that they were too negative and kept putting on me. They flipped and threatened to kill me and come to my house. After the other mate called the police,the following day i got a nasty text off the one that threatened me(they are mates) I now know that they both got together to call the police on me. I should've just sat down and discussed my concerns with them both. They said I was naive and obnoxious. Perhaps I am.