This is a rather interesting insight into the behaviour of the Nomad race. They are not, by nature, defensive creatures, and are not fighting for their own existence, as much as for the destruction of humanity...
Yet they only kill those they perceive a threat? Very interesting indeed. Excuse me for referencing a non-freelancer sci-fi series, but this reminds me allot of the Borg...intent on enslaving all life, but will only interact with ships or individuals they perceive as a threat, or in the way of them achieving their goals.
But what are their goals? What do the Keepers do other than cause mayhem? Perhaps, they should set up a trader wing, to transport alien artifacts and other materials to the nomad gate construction sites. This would allow Sirius not to be constantly on the defensive, and allow some interesting RP to develop.