It appears your ex-boss

actually didn't know the real important reason why you left the company, that is why he still was trying to hire you back.

If I were you, I probably would start a casual conversation with him, ask: Is that sucker still there? How is he doing? Based on your ex-boss's feedback, then you can decide what to do, most importantly you will give him a strong indication that you and that sucker couldn't get along. If your ex-boss decide to get rid of that sucker, then it is good idea to go back with a brighter future.