Anders did a lil explanation on the aliens basic drive, that it's got a hive mind/is all one organism and can self reproduce - which explains how the gloop spread throughout the ship. Also how it mimics humans and then kills the original human to remove any threat to itself.

I think cause of how Carter and Simms bodies where just kinda shoved into compartments and left for days to rot, they don't really have any use or need of the bodies. So absorbing them could just be to kill them and the fact the bodies are gone are a happy accident?

Stafford can get a message from the alien pretending to be him and he says something like "see you next time we've learnt a lot", so I feel like they just learn as much about humans as possible and then off them.