I feel like the only time cGrian really expresses genuine remorse/guilt is when he loses control of something or something doesn't go the way he planned. I think this most notably comes into play in third life. Obviously he accidently kills Scar and kicks off the deaths way earlier than he intended to, hence the swearing himself to Scar. And later, in the finale, instead of him slaying Scar in a vengeful blaze of glory like he likely expected it to play out, Scar forfeits which he has not mentally equipped himself to be okay with.
In a similar vein, not once throughout double life does he apologise nor shows remorse for the way he treated Scar, but at the end when his plan to lure the warden spirals out of control, finally does he apologise (for something that, comparatively, Scar probably isn't all that pressed about).
In past life, he actively seeks Martyn out to apologise for accidently blowing him up despite having moments earlier literally planned to possibly kill like, 2 or 3 people. I also, think he does feel guilt and regret regarding Pearl's death. Though to be frank its probably as much if not more regret regarding the early loss of what he views as a powerful asset and cognitive dissonance relating to his caricaturized perception of Pearl being proven incorrect than it is remorse for his actual mistreatment of her.
Maybe I'm just crazy and cherrypicking but idk is smth clicked in my brain when I was thinking abt this