It's funny because the game tries to portray Abby's dad as this super-good person who didn't deserve to be killed by Joel by having him save a zebra's life in a flashback. This is the same guy who was willing to cut open a 14-year-old girl's brain because it might hold the key to a vaccine for the cordyceps plague.It's interesting how a lot of people are unfazed seeing fictional (or even actual) people being brutalized or killed, but completely lose their shit at the sight of an animal being abused or killed.
It probably has to do with animals being viewed as generally "innocent," with even bad behavior being excusable because of illness, injury and instinct, where human beings are notorious for their willful nastiness. A person trying to kill you is a shithead. A dog trying to kill you is either hungry and desperate, or well-trained and obeying a command. It's easier to feel bad for the dog if he dies.