I believe Macheath's biggest flaw is his lack of emotion or regret for how he's lived his life. He shows no reaction when Polly reads the list of charges against him. He's committed murder, forgery, arson, and burglary in an eighteen-month period. His womanizing is just one more of his immoral crimes against society and humanity. It doesn't matter to Macheath if what he is doing is wrong; that's not even a consideration. He doesn't stop to think about the victims of his crimes. I think today he might be called a sociopath.
MacHeath's Achilles' heel is his sexual appetite. Juggling his various conquests sometimes is not as successful as he would wish. When his father-in-law Peachum gets sick of his philadering ways and wants him out of his daughter's life, he turns MacHeath in for his long list of crimes. Had he curbed his, ummm, enthusiasm, perhaps he would have gotten away with his womanzing longer.