Okay, so Gabriel's parents aren't exactly candidates for any parenting awards, so while we're definitely not condoning his decision to murder them, we do get his frustration with them. One of his biggest issues with his parents is how they say one thing and do another. Take how they treat his brother Vernon, for example: His mom calls Vernon life's joy to everyone's face, but in reality, she despises him.
Time and again, Gabriel sees his parents living double lives. They pretend to be church-going, God-fearing, loving parents—all while hating their children and beating them for the slightest disappointment. When Gabriel finally snaps, Finnigan's question to him isn't why he's killed his parents, but instead, why now. It's clear that Gabriel's alter ego can see how destructive and crushing Gabriel's parents are to him, and for someone with so much darkness lurking right below the surface, it is kind of a wonder it takes so long for them to meet their demise.