He's done that since the beginning, everyone kept telling him "You're just like your mom", "You're an abyss" but he didn't really understand it until it became a coping mechanism and a way of getting revenge, like when he finally snapped and told his monster of a gma to fucking die already or accepted Yuri's declaration of live.
Like his mother, he doesn't consider the future implications of his actions, he acts on his feelings in the moment, whether it's fear of being alone or anger. He's a long way from the kid he used to be, the kid who unintentionally allowed a domino effect of misery to spread and continue because of his ignorance, he knows exactly what he's doing, he is everything his mother wished she could be.
Yuri doesn't love him, she's obsessed with the image of him in her mind, and Nagi too for that matter.
I have a feeling after the next chapter, she's going to not be able to delude herself anymore.
Mao deserves death, the problem is Reiji should be focused on helping Nagi get better, killing Mao will solve nothing except give himself temporary satisfaction... and then get sent to prison and hanged, leaving Nagi alone and at the hands of another monster.