If you take into account all routes the fact that everyone can die rather removes alot of suspects. Also pretty sure there's a second rat named hypnos that is thanatos's brother and thats who's been able to steal from the lab, move around unseen and talking to sal in the night. Having both thanatos and hypnos as names can't be just coincidence considering their mythology. Also would be fitting with the way oz talks about his sister as her being the most forceful out of the three since in the mythos its thanatos and hypnos sisters who are all about violent death.