It has been a long time since I last browsed through the questions here haha. I'm back! :D "Hắn" alone implies a bad man. So yes, it's used to show someone's disrespect to the person being mentioned. But sometimes it can also be used to show intimacy. (Sorry you're learning Vietnamese, Raph! :D) I've heard my dad use "hắn" referring my mom to me. :D And I also heard some of my schoolmates using "hắn" when they refered to another close friend of them. (They're from Central Vietnam). But in your sentence, yes, it's disrespectful. (Sorry for confusing your mind. :P) Adding "ta" doesn't lead to much change in the meaning of "hắn". You may encounter "chị ta", "anh ta", "cô ta", "chàng ta", etc. later on. Nice day! ^_^

"hắn" or "hắn ta" indicates only a man and you use this singular person pronoun to express disrespect and imply that he is the bad man "bọn hắn" is plural person pronoun of "hắn" or "hắn ta"

Those are to call the third person who is a (one) male disrespectfully. with or without "ta" doesn't change the meaning. Hope it helps you :)

"cô đi cùng với hắn, đúng không?" and " cô đi cùng với hắn ta, đúng không?" both of them are true!

"...ta" means personal. "Tôi - ta", "chúng ta, hắn ta, cô ta, nàng ta,...". In English, "hắn ta" = "anh ta, chàng ta, cậu ta" = he/him, but yes, it's kinda disrespectful, not like the others.