Cal and Niko's refusal to forgive Kalakos in Doubletake. He is not Easily Forgiven in the tradition of so many Disney movies - the exact opposite, in fact. The entire book is basically one long, brutal Calling the Old Man Out for abandoning his son. Of special note is Cal's conversation with him after Kalakos doesn't understand why his son stabbed him for preventing Niko from reaching Cal when he was almost certainly dead.
Cal: Nope. I don't think you were wrong. Keeping Nik safe is it. Number one priority for me.
Kalakos: Then you see. Then why won't he? I want my son back. We were making progress and now this. If it had been you I'd have held back, with Niko already beyond reach and gone, you would've understood.
Cal: Oh yeah, I'd understand. If you'd tried to hold me back from getting to Nik, no matter what the odds, I'd understand you'd be breathing through a bullet hole in your throat. I'm going to cut you a break because you did save Nik at least once. But I know you won't get it.You look like Nik, but you're not half as smart, and wanting to invest in emotion for your son now? Is 'too late' not in your vocabulary? But since you want to try....So listen, Kalakos. Concentrate. I'd do anything to save my brother. Anything. And he'd do the same for me. He has done the same. Think it through. He risks his life for mine, Then I see the mess he's gotten himself into while pulling my ass out of the fire, and I'm right back in after him. Same risk. Same mess. Then back it goes again. Vicious circle, I think they call it.
Kalakos: That's all but suicide.
Cal: And if we're too far from each other and it's too late, vengeance has nothing to do with the Lord. Cain was wrong. I am my brother's keeper. And Niko's mine.
Kalakos: That is suicide, plain and simple.
Cal: You said it yourself, Father Kalakos. Niko is who he is because of me. I am who I am because of him. For me that won't stop. I'm terminal and Nik is my chemo. He keeps me human and he keeps me sane. Niko, I don't know what he would've been without me. Not you. Absolutely not you. Better off? Maybe. Able to love, like he loves Promise? To trust, like he trusts Goodfellow? I don't know. And neither does he.
Kalakos: It's insane. Friends and family members grieve and go on when people die. They don't jump into the grave after them. People live on.
Cal: Yeah, people do.
Certain monsters and heroes don't.