Well, Zach almost kills Bryce. It's not surprising, either, considering the tension that bubbles between them all season long. In the penultimate episode, "And Then the Hurricane Hit," we discover just what happens between the two of them that leads to Bryce's last night alive.
Through flashbacks we know that Chlöe had already told her ex about her abortion, which Zach helped her get through. Zach's involvement in Chlöe's life majorly rankles Bryce's ego, which is what sparks a football injury that leaves Zach's future college football career dashed — during the Homecoming game brawl, Bryce gets his revenge on Zach by purposefully diving into his former friend's knee with his football helmet, instantly shattering it.
Ani's repetition of the phrase "violence begets violence" in season three ends up applying to these two in particular, when Zach teaches a physical lesson to Bryce on the docks after being released from the hospital and trailing Bryce post-game. Despite only having one good leg, he proceeds to beat Bryce nearly to death.
During the scene, Bryce screams about Zach "taking away" his future by convincing Chlöe to dump him, and bitterly asking him if he "f*cked her," proving exactly what we've known all along: Bryce hasn't changed one bit; he still sees women as objects, as opposed to people. Zach's response? Breaking Bryce's leg. Woof. But, in a feat of self-control, Zach limps away before doing any more damage to Bryce. When he leaves him, Bryce is still alive.