I think Alex knows she crushed on her escort knights before and that she was saddened when they left her, and since it's happened so many times he probably doesn't want it happen again which explains his hostility and why he says "if you betray my sister I'll kill you" rather than "if you try to...
Electricity won't be working for very long after a zombie apocalypse...also nuclear reactors will explode and release a bunch of radiation...also how are zombies able to generate energy to move if their body is dead? but I guess I shouldn't be thinking about it too hard since it's just fiction