At first glance, Ari seems like your typical Eraser—he's a sneering, bloodthirsty wolf hybrid who wants to destroy all of the flock members. The flock members are shocked to see him, though, because it turns out that he's Jeb's son and he used to be a normal, sweet kid. Something terrible has happened to him in order to turn him into the monster he is when we meet him, but even though he wants to kill them all, Max kind of feels sorry for him and mourns the child she once knew:
Despite everything, despite the fact that I could cheerfully have kicked his teeth in for what he had done to Fang, I did feel a pang of pity for Ari. It was true—once we were out of the School, I'd never given him a second thought. I didn't think about why Jeb had left him or what had happened to him.
"Someone did terrible things to you because Jeb wasn't there to protect you," I said quietly. (130.8-9)
Ari's hatred of the flock members is also more complicated. He doesn't want to kill them just because it's a fun sport—which is why the other Erasers are hunting them down—no, Ari actually holds a pretty serious grudge against Max because she always got more attention from Jeb. At one point, Ari goes into a jealous/murderous rage when Max goads him with this fact:
I spit at him. "Daddy always loved me best!" I hissed, and his face darkened.
He took a fast step toward me, paws coiled into fists, but was pushed along by a rough, hairy wave of the other Erasers. (117.16-17)
In the end, even though Max comes to hate Ari and ends up killing him, she still feels sorry for him and the painful circumstances that led him to this end. He's become a monster, but not by his own doing—and Max's ability to recognize this shows how true her heart is.