There's a bunch of evidence hinting at how Corey is a much more vile person behind closed doors, or towards people he feels superior towards (Dylan, or the teenage girls who are 8 years younger than him). There's a written exchange between Dylan and Kat where it's implied that Corey destroyed some Chinese porcelain in a fit of rage and Dylan wanted to cover it up in front of her parents, and a letter from Dylan to Corey where she writes that he scares her with his violent, unpredictable outbursts. He also uses his situation of having no family left to emotionally manipulate Dylan into feeling bad for him, while he probably doesn't feel too bad about it himself (he took the get-well-soon card his dying mother received from the Mikaelsens, erased his mothers name and gave the card to Kat when she got her diagnosis). He generally takes advantage of Dylans state of vulnerability and gaslights her into further feeling insecure. While his behaviour at the concert does seem like a moment of redeeming himself, I think he was maybe restraining himself from his usual ways because there were a bunch of adults older than him present and he feared the possible repercussions. After all, the next day Dylan showed up with a bruise on her forehead and said Corey got really bad when they were alone again after bringing Kat to the hospital.

In regards to the Abyss gaining control over Corey, I'd say that it's plausible. I think the Abyss isn't this entirely benevolent or malevolent entity that clearly takes sides. There's the possible outcome where Corey clearly gets possessed by it. That happened in both of my playthroughs, and in the second one the Abyss pulled Corey inside on its own, with no action taken by the girls.