Some TMNT stuff really isn't for little kids.
When Arnold was a wild young kid, he got his first tattoo and formed the Purple Dragons gang. Life was good for Hun until he met a young woman, got married, and settled down. He quit his more outwardly violent gang lifestyle. When his wife got sick, Arnold fell into alcoholism and became abusive toward his son Casey Jones after the passing of his wife.
Once he'd seen Casey in the hospital following Shredder's assault on him and overheard Angel saying some disparaging things about him, Arnold tore off his shirt and punched a mirror in the hospital bathroom, vowing to never drink again.
On his way home from an Alcoholics Anonymous meeting, Leonardo (who was brainwashed by the Foot Clan at the time) approaches Arnold and tells him to come with him. Arnold, recognizing Leonardo as one of the mutants his son hangs out with, gets angry and tries to attack, but Leonardo easily defeats him and knocks him unconscious. When Arnold comes to, he's tied to a chair in a dark room. The Shredder stands before him. Arnold asks who he is, and Shredder tells him: the man who's about to change his life. Shredder reveals he knows that his son Casey was injured. He asks Arnold how he expects to pay for his sons medical bills, how does he expect to care for him once he leaves the hospital. How does he expect to protect his son from the men who hurt him, who are still out there? Arnold says Shredder doesn't know anything about him, but Shredder says he is wrong. Shredder knows Arnold has been languishing at the bottom of a bottle for years and that his old gang the Purple Dragons are divided and leaderless. He tells Arnold he wants him to retake control of the Purple Dragons and work under him. Arnold says he's not the Hun anymore. The Shredder tells him he could be, and opens a briefcase with a vial full of mutagen. Arnold knows its the same stuff that created the Turtles and wants nothing to do with it. Shredder tells him that it won't transform him, just make him into the man he used to be, better than the man he used to be. All Arnold has to do is remember who he's working for. Bulked up from the effects of the mutagen and his hair bleached back to blond, Arnold (now operating as Hun again) approaches the Purple Dragons headquarters. Inside, Angel is telling the Dragons that they're not going to get involved in the turf war with the Foot and the Savate. One of the other Dragons says maybe they should put it to a vote, but Angel says she's in charge. Arnold steps forward and says that's not the case anymore. Angel orders the Dragons to get Arnold out of there. Arnold makes short work of them and forces Angel to relinquish her leadership to him. After retaking control of the Purple Dragons, Arnold went right to work settling debts and doing the Shredder's bidding. Arnold later goes to visit Casey in the hospital. He tells him he's quit drinking and got a job, and that things are going to be different now. He tells Casey that he may be able to come to work with him one day. He gives him a gift and leaves. Casey opens it to find a brand new hockey mask. Arnold tells the AA group that he still thinks about having a drink every now and then. But then he thinks about his new job, and how good he is at it and how much he enjoys it. He thinks about his son and how now he'll be able to provide for him and he's able to resist that temptation. "One day at a time" he says.