Pike approaching Octavia after A.L.I.E is defeated and him giving her a head nod feels like he was telling her that he was ready for her to kill him now and him dying on the floor looks like he's accepting his death.
Though his head nod could also be him telling her your welcome for saving your life or he was just showing her respect and he really didn't expect her to stab him. Even then most viewers saw him approaching her as him expecting that she will just forgive him and no longer wanted to kill him because of saving her life a moment ago.
Though if the letting Octavia kill him interpretation is true it could mean that he was possibly sorry for what he did to Lincoln when you consider that it didn't look like he enjoyed killing him and he even seemed to respect him despite the fact he was a grounder by how he told him that he will keep his word and take care of the Grounder prisoners before pulling the trigger or he was sorry for what killing Lincoln did to Octavia since she used to be one of his students and being Bellamy's sister who he also cares about. But it could also be that he was accepting that no matter what he was going to punished for all the things he did by either her hands or Indra's and he lets it be by Octavia's hands. Although it is possible that if Octavia or Indra didn't kill him the other Grounders would and it is known how they prefer to execute people.
Related to Pike what would have Clarke and the others have done with Pike after A.L.I.E was defeated if Octavia hadn't killed him?
One theory is they either would have turned him in to the Grounders which is what they did originally or would have imprisoned him or executed him themselves.
But the show itself seemed to imply he would have ended up on a redemption arc similar to Kane's in the first season by how his flashback story and his alliance with the others seemed to be gathering sympathy for him despite his actions and the show even did this for Murphy who was a sadistic bully who killed people and tried to kill Bellamy in the first season. Because of this it is possible that the others would have tolerated working with him to try and find a solution to the nuclear problem and of course the producers were originally going to bring him back for season 4 until his actor was hired for another show and Octavia walking away from everybody could have been her not being able to forgive him but also unable to kill him or she still attempts to kill him but he survives or he was supposed to collapse from the wound he got in the last episode that wasn't adressed again in this episode.
There was some speculation about whether or not A.L.I.E was telling the truth about the nuclear plants and if she was just lying to stall Clarke from pulling the kill switch but the plot for season 4 confirms she wasn't. Though telling her the truth seems like it was stalling either way since she could have told Clarke about the nuclear plants when she was looking for the kill switch and only decided to tell her when she was about to pull it.
