Fuck Wit Dre Day The Chronic Dr. Dre

I was a mark 'cause I used to hang with Eazy

Actually, "mark" is a derrogatory term. Dre is saying just by affiliation, he was a loser.
In this case this is a dig at rapper Easy-E. Dre is saying that people thought of him as a "mark" (a term used to describe someone who is easy to take advantage of) because he used to be affiliated with Easy-E and this was bad for his reputation.
He was the target of negativity and/or guns because of his affiliation with rapper Eazy-E