And Indiana Jones was in his late 20s banging Marion when she was 13. Wolverine and Deadpool are murders. Darth Vader kills children and still gets to Jedi heaven. Cinematic stories constantly give us anti-heroes or flawed people with we can't help but like. Redemption arcs are one of the core storytelling tropes ever.
It's not like Eddie's actions haven't gone unanswered. He witnesses a traumatizing brutal supernatural murder, all of Hawkins is hunting/blaming him, and he's ended up in the Upside Down. He's not having a good time, so in essence he's paying for his sins. He's started to break good, so if he makes it out alive, he'll be redeemed. They've done this pretty much every season, first with Steve, then with Billy. Granted, Billy never fully gave up being a douche, but in Season One we hated Steve for corrupting Nancy until we suddenly didn't anymore.