Bad boy boyfriend or cheating ex? PLEASE HELP ME , MATURE ANSWERS!?

I was with my ex fiance for 4 years until he slept with someone else, it broke my heart. I loved him so much and gave him my all. We havent been together in over a year,but we have a load of mutual friends. I recently started seeing a man who has a bad reputation & is a former boxer with a history of violence, including road rage and drive by shootings & drug dealing among other things. He is really good to me though and everyone loves him despite his reputation. He is really into me as he tells me all the time that he loves me and will always be at my side when we are out. I know it sounds crazy that i am with him but you cant help who you are attracted too.My ex is going nuts at me for being with him, i dont know why he cares when he didnt care enough about me when he cheated. He is asking me to go back to him but i dont want to feel that pain he put me through again, even though i love him. What would be the best thing for me to do, As i think i am falling in love with my boyfriend but i still love my ex?! My boyfriend has the worst history of them both but has treated me better than my ex & is talking to me about marriage.Please help and please dont attack me as i am just looking for advice. thanks!