IMO Pike's reason for killing Lincoln was pretty weak. There were other ways he could exert his power without putting a bullet in the head of someone who had been peaceful.
Lexa had to kill Gustus because he was a trusted ally who went behind Lexa's back and tried to poison Sky People. They have their customs and their ways and that's what she was sticking to. Obviously it was terrible to watch and we the viewer should feel badly for Gustus. He did it because he wanted to protect Lexa and thought she was in danger.
Lincoln sacrificed himself to save his people, and Pike killed him in cold-blood (this article
here is pretty agreeable IMO).
So similar reasons (betrayal/treason/whatever according to the person in charge), but not the same.
PS Adam-you saying Lexa was only there for Clarke's development is not bashing, that's an opinion. You don't have to love every character, of course - that's why we're here to discuss them and tastefully/respectfully tell eachother why we agree or disagree.