"You're either the butcher or the cattle."
That was the reality Gareth had to learn the hard way when those bastards took over. That in the chaotic world they're all surviving on, there is no such thing as kindness anymore. They wanted to save people, help survivors. Instead, like some sick joke, some monsters decided to take it all for themselves.
Cruel, heartless men used their kindness against them by taking all their resources, locking them up in train cars, beating them, and r@ping their women, including his mother. He promised that they'll get the place back, and they did. Gareth lead them and they killed those monsters EXCEPT for the leader. They locked him up in the same place he kept them as killing him, to them, meant mercy. And he certainly didn't deserve that.
They reclaimed what was theirs, but these violent men had forced them to do what they do. After so much suffering, Gareth, his brother, Alex, and their mother decided to be part of the butcher shop so as not to be the cattle. Eating people saved resources and saved THEM for future possible threats again.
Gareth knows it's wrong but he feels justified by this. He doesn't want his people, his family, to go through that ever again. This world had shaped him, CHANGED him. Not everyone who survived the outbreak were good people. It's either kill or be killed.