Lexa is one of the most complex characters television has offered us in a while. I can’t remember the last time I felt so challenged to understand a character. Lexa did what she had to do for her people. The easy decision would have been to reject the deal, fight by Clarke’s side and risk never getting her people out and her more of her people dying in the process (things were not going according to plan, mind you)
and then maybe having some sort of future with Clarke, whom she just might be in love with.
But she’s the Commander. She’s a leader. She broke her own heart to spare the lives of her people and ensure their safety. She sacrificed her personal happiness. She made the hardest call she could have made. I have no doubt in my mind she cares deeply for Clarke but those feelings had no place in making this decision. And THIS is precisely why Lexa doesn’t want to care and at least made some attempt to not care about people because one way or another, she loses them; Costia, Gustus, an entire fucking village and now Clarke. It comes with her position as Commander.
"Victory stands on the back of sacrifice." That’s pretty much the lesson of the season, one Lexa understands to an unbearable degree.