The killer would be modding the Puppeteers.
Total cowards but incredibly advanced. You would have to give them a high tech start and frankly that would not be enough to equal their real power. Make them a neutral and put them in an unconnected system. If you have the bad luck to enter their system, well... just stick your head between your legs and kiss your a** goodbye!
They want no-one to know where they live and will destroy you "down to the Last kitten" to hide their secret.
It would never occur to them to capture and use your people-- much too dangerous to allow near the peaceful Puppeteers!
So, if you know they are around, best you NOT look for them. In fact, encourage your enemies to look for them! This might be a major strategy when the Puppeteers are involved. I'd much rather fight the Kzinti!
Kim