Like ex's best friend

I like my ex's (2 weeks ago) best friend and i know his best friend loves him as a friend a lot but my ex tried to get with my friend but it didn't work. I feel now my ex is trying to get with my best friend and theres a bit of a history between me and the guy I like I'm going to tell him I like him but I dont want him to think I like him to get back at my ex so what do i do?