Why doesn’t Titania just kill Stag herself, or get Lily or Ella or another drone to do it?

She could do it herself, but I’m afraid the truth is that she is just something of an ice queen – she won’t put herself in danger if she get other people to do it for her.  As Ten explains, Titania doesn’t puppet the drones, she just suppresses their memories and if she isn’t telling them what to do their behaviour is mostly instinctive.  They don’t really think too much, so she doesn’t trust that they wouldn’t fall into one of Stag’s traps.  And Lily and Ella she actually does have a bit of a personal attachment to, so she resists sending them on any very dangerous mission.  So she uses Ten, and getting Jen to override the locks and cameras is a bonus too.