Peter was a prisoner of Anthony Ivo. As part of Ivo's experiments, he was subjected to radiation poisoning so Ivo could test if the Mirakuru would cure cancer. Ivo justified this by saying, "The essence of heroism is to die so that others can live". Peter later credited this adage as the only thing that allowed him to hold on to hope.
In "Birds of Prey", along with many other prisoners, he managed to escape during the freighter riot. However, when those loyal to Oliver planned on surrendering Hendrick Von Arnim to Slade Wilson, he wasn't supportive of the idea - as he believed it wasn't their right to decide whose life was more valuable - but he eventually concede and let the rest of the group hand Hendrick over without a fight.
With Oliver Queen, Sara Lance and Anatoly Knyazev, he headed to the Japanese military submarine with the hope of using it to escape the island. However, the submarine had run aground and could only be dislodged by firing a torpedo into the seafloor beneath it, an act that would require a pilot to steer the torpedo into the ground. Peter volunteered to be the pilot, given his inevitable death from the radiation poisoning he had received and his desire to save the other three. Before he stepped into the torpedo, he told Oliver Ivo's words, which Oliver would remember during Slade's attempt to take Starling City. Peter's sacrifice was ultimately successful, the blast managing to break the submarine free from the seafloor.
Peter was a selfless individual, shown when he volunteered to take a suicide mission to take down Slade, albeit part of this was because he was going to die shortly anyway, from cancer which was caused by Ivo's experiments.