Thanks for the interesting question. I’m not sure when I wrote about it, but yes, I do think it was just a moment to shock the audience because there is nothing about that choice that makes much sense from a military point of view. To say it weakens Lexa is an understatement. By walking away, she negates her own reason for being at Mt. Weather: to defeat the enemy of her people. Taking a deal with them is against everything she has been working toward and Lexa proved herself to be, until that moment, a most logical and practical person. Taking the deal left her and her people open to future attacks either by the Mt. people or by Skaikru, if they took over Mt. Weather. I believe the decision was made by the writer to create a moment of shock for the audience and to build the scenario we got in the first half of s3.
But I’ve thought too, knowing now that Lexa was a cyborg, her mind the host of an AI database that gave her access to the thoughts/memories of her predecessors (though this was dealt with weakly in s3 – “Lexa didn’t know” – Lexa SHOULD have known): what if she was influenced by the thoughts/memories of Becca? A woman whose invention took some 6 billion lives. What if Lexa was, essentially, ‘programmed’ to save those lives out of Becca’s guilt for killing off most of the human race? Instead of acting on her military instincts, Lexa/Becca made a choice to save as many lives as she could. It also underlines Lexa’s point that she made the decision with her head, not her heart. If ‘Lexa’ had not had Becca’s influence, she probably wouldn’t have broken the alliance and stayed with Clarke.
I’ve no idea if the writers had this in mind, but I think it gives a bit of sense to Lexa’s decision. Hope my thoughts weren’t too cluttered. 🙂