I can't agree with the position of some of you regarding evil thalan, I understand that they don't destroy the mind of the host. IMHO, they are pure mental energy that can interact with a woman to get her pregnant and be their own father.
I would think more than they are jealous of the people with body and they want one, maybe they pay a price leaving the bodiless existence so they can be killed.
In the timeline of the Atlas, near the end I read:
Agyra: One of the Inner Circle of the Alliance is assassinated by a prist of the Silver Dawn.
Although it's not clear if it's one of the Brotherhood or maybe a high rank Jenaar, I find more interesting that the thalan pays the price of the true inmortality to be "real", that turns them into a less pure evil being but more human.