I think Cain regrets killing Abel. And not in a 'now I'm in Hell, so that was a mistake' kinda way. I think he regretted it immediately because I don't think killing Abel was necessarily his intent. After all, it's not like they had any idea any of them COULD die. No human had ever died before then, only animals. I see it more as something done in a fit of anger, when all rational thought had left him. Thus, when that fog of anger lifts, he's left with the realization of what he's done and how he can't take it back.
Still a fucking overreaction to being snubbed by God, but I see it being like THAT scene in Hereditary (iykyk). The way the gravity of what he did sinks in and him then feeling the full weight of it crashing over him. Followed by the immense panic laced with dread.
So it's a guilt he carries with him for, like, millennia even in Hell. I don't think he believes he can ever be redeemed - both because he was marked by God for punishment and because he doesn't believe he deserves it - but I think he'd want to at least TRY and apologize to Abel. He's just REAL bad at it.
