Potato Bomb

OK just answer me this, I love this show but sometimes it makes me wonder "why", idk if the writers are getting lazy or what. Why did Lexi have to die anyway if the Volm came back with their ship(s) and helped Tom, they could've just destroy the power source on the Moon by shooting at it or something instead Lexi just killed herself anyway that didn't make any sense to me and it's not very realistic or logic :/ also how could a thing as important as a power source to the Espheni be so unprotected? not even a turret or nothing.. thats very unlikely but oh well... they had to kill the main character's daughter to make it dramatic I guess, I wish Lexi had not died and just lived and became human without god like powers instead. What do you think?