Text from "After Twenty Years" which best supports the idea that Jimmy Wells is a good man is "Somehow I couldn’t do it myself, so I went around and got a plain clothes man to do the job."
Between two painful alternatives, Jimmy had to make a difficult choice. On one hand, he could have let his friend Silky Bob escape and gone on his way but Jimmy as a police officer would have been guilty of failing to do his sworn duty. On the other hand, he would be guilty of betraying his friend by causing him to be arrested.
Being a law-abiding and moral person he arrests his friend and he could not ignore it.