Where does it say that if you develop psychic powers you can get psy ratings as elite advances over and over again? Otherwise your basicly going to elite advancing being the whole psyker career if you do that. And do you get that first psy rating as an assassin?
Dark Heresy, Ascension, pg. 67
Interrogator- Ascended Traits: The Psyker's Gift
"As with their masters, many Interrogators are psykers and develop their powers under their masters tutelage. You must already be a psyker to select this trait. You may select the Minor Psychic Power Talent as an Elite Advance for 500 xp, the Major Psychic Power Talent as an Elite Advance for 750 xp, or the Ascended Psychic Power as an Elite Advance for 2,000 xp. You may also increase your current Psy Rating by +1 as an Elite Advance for 1,500 xp. The xp cost of the Ascended Psychic Power talent may be reduced in the normal fashion (see pg. 125)."
And now, I will probably answer my own question that I had at the beginning of this thread. If a Moritat Assassin gained a mutation that game him psychic powers (Wyrdling mutation or Nascent Psyker elite advance), he would either off himself, or turn himself into the authorities of the Black Ships.
However, if the assassin had proved himself valuable enough (or pure enough) to his master, and he survived until Ascension (no small feat), the Moritat could become an Interrogator, thus gaining more psychic power, and using a force sword to the best of his abilities. He could not become a Death Cult Assassin (as I previously thought), but he has at least one way to keep alive.