@verius Kosh Naranek is more or less said to have done it out of caring for those races and it's implied he had some knowledge of the Shadow's corruption of EarthForce, and his "death" essentially becomes an act of sacrifice.  Ulkesh (his replacement) on the otherhand, basically resents humanity (it's implied he blames Kosh's death on Sheridan) and the only use they have to him is fighting the Shadow, so they better hop to it.