I am very depressed and feel guilty about my ex dying?

Stop feeling guilty. Focus on the good things about him. Poor guy had a bad life very unfortunate. He is partly to blame for being in a gang and addicted to drugs. His actions probably were the cause for his stabbing. You are who you hang out with. You hang out with good decent people you will have good decent friends. You hang out with people who are crazy people will think you are crazy as well. You gave him love and that is all that matters. Next time have better judgment on guys. He is most likely in a better place. Focus on the happy memories that is all you can do.