What Octavia did at the last second was unforgivable. I know why and he's done bad things in the past and blah blah blah but to just kill someone in cold blood after they protected your people? I know it was partly his fault for the mess in the first place but by this point in the show everyone has done something to mean they should take part of the blame but she just can't see that.
I know he killed her soulmate but actually if Clarke can forgive that guy who killed Lexa, Kane and Abby can forgive each other for their respective roles in what they did under ALIE's influence, Bellamy can forgive Clarke, Monty can forgive Jasper, etc. Octavia's character is weak if she can't see past her anger. And killing Pike is not going to make her feel any better - it doesn't bring back Lincoln and let's face it she has a vendetta against about a million other people as well, so is she just gonna go on a killing spree until everyone who was even slightly involved of the death of her boyfriend is dead? Cos that'll be the end of the show.
But yay, at least ALIE's gone.