Contrary to the other posters, I'll say that it is perfectly fine for a Moritat Assassin to be a psyker and use a force sword. For one, there are exactly zero sources of fluff that suggest the Moritat, or even Death Cults in general, hate psykers. When no members of a group can be psykers, the career rank says so – for example, with the Sister careers. The Moritat have no such restriction.
Furthermore, as to a force sword leading to fame and people hunting you – you're an assassin, and force swords don't look any different from any other kind of sword to people who aren't familiar with them. What people will see is that you stab people and they die, which while impressive is not altogether unusual. Thus you'll be no more famous or hunted than any other assassin.
In short, yes, you may be a psychic Moritat Assassin with a force sword. It isn't even worth vetoing on the grounds of power level, as its not that powerful a combo in the first place; you'll be just like a normal assassin but with a few risky tricks and a psy rating of 1 (there isn't any way for an assassin to get a psy rating of 2 before Ascention, at which point power levels cease to matter anyway, except for elite advances which are always GM's discretion).