Once the 9/11 operator told him not to follow the guy - and Ashtray knew he was being followed - obviously he was not going to rob any houses since they were on to him.
At that point Zimm should have just walked away since there was no evidence a crime had been committed.
Sometimes listening to the 911 operator will get you killed. Its not always a good idea to listen to them, nor do you have to.
Also, I think Martin would have attacked Zimmerman regardless. It really didn`t matter. Trayvon made up his mind to attack. He was that kind of person.