One of the reasons why I never got behind the Lexi/Clarke ship is because of their ridiculous choices. I don't think Lexi would have chose mercy if she wasn't trying to maintain favor with Clarke. Clarke doesn't seem to take into consideration how dangerous it is for Lexi to not take some sort of action. Grounders respect fierce and powerful leaders who protect their own. To let the death of 300 go without any consequence, makes her look weak. It's the same reason why she executed Finn. You don't need a full blown war but there needs to be some consequences. Indra is just trying to protect her. Leaders are replaced all the time.