most likely your bottom box is empty like 5 out of seven of mine were today....placing another box on top....I think they will probably keep going up and ignore the bottom.
They weren't killing them. When you pulled the cover off, the comb tore and exposed the brood. They were most likely drones. Mine are building drone comb like that. As long as there is no honey band on top, they will move up into the third deep. You have bought some time but you'll need to watch for swarm cells as soon as it warms up.Maybe you are right what i saw them tending were white they looked like really big maggots but very very white. So maybe they were killing them not tending them. I guess I would have to put them on top of the bars in the empty body but i do not know if i really want a hive that has three bodies but considering I want hoeny I may not have a choice I do not want to split this hive unless I have too.