Varnias Tybalt said:
Are you sure that you're not just correlating imperial assassins with ninjas subconsciously because imperial assassins often wear black bodygloves and masks?
Now I can't really say that I think any of the imperial assassin temples resemble anything in a kung fu or ninjaesque way. We have the Cailldus Temple, whose assassins are masters of disguise and deception (but that's pretty much all they do), we have the Vindicare which are specialized snipers, we have the Culexus which are Untouchable anti-psykers, and we have the Eversor which are psychopathic "one man armies" fighting with guerilla and terrorist tactics.
Make no mistake, im sure that all of these have had extensive CQC-training and are proficient with many types of close combat weapons, but I fail to see why extensive CQC-training necessarily have to be oriental style martial arts, when they could just as well be simple, yet extremely effective military versions of martial arts?
My point here being: why would an assassin ever employ mythical kung fu/ninja stuff like the "five point palm exploding heart technique", when a slit throat or a snapped neck is just as quick, silent, effective and way more fool-proof than fictional "chi" usage and pressure point manipulation?
It's not that I dislike oriental combat myths and such (I am a huge ninja buff myself), but I think it's a bit stereotypical in a way when everybody thinks "badass combat monster" they always think of oriental martial arts.
An imperial assassin going more along the lines with badass "military-grade effective", rather than "elaborate mythical ninja effective" is by far more cooler and a lot more along the lines of the 40K-mindset, with all it's modern military romanticism and such.
Besides, we all know how world war 2 went. The US faced of against Japan. That's an entire nation of ninjas and samurai if you are to believe all the myths and martial arts romanticism. In fact, many of the japanese soldiers actially carried katana swords during the battles. Now which side won the war in the end? The mythical, uber ninja/samurai warriors, or the normal soldiers with guns, battleships, tanks and bombs? 
Well, to be fair, the Japanese also had normal solders (except they had a bad habit of not giving up nor surrendering no matter the odds), a whole mess of civilians who would also fight if invaded, fighter planes, suicide bombers, normal bombs, lots of guns and bullets, and those katanas as well (tanks, to my knowledge, didn't really didn't play a major role in the Pacific battles). Unfortunately, there's no martial art move called supper atomic air strike (well, maybe there is in DBZ) and two atomic bombs will always trump a bunch of guys with katanas any day, no matter their training. They were seriously giving us extreme problems, katana and all, which is why we decided to bomb the ever loving snot out of a whole generation of Japanese civilians. It was the only thing anyone could think of to get them to stop.
As ninjas go, they didn't really use mystical kung-fu (and kung-fu is Chinese anyway). They used psychological warfare, misdirection, dirty tactics, and the all famous Sneak Attack to put sharp pointy things into other things that scream and bleed... or poison in their food, or put a poisoned sharp thing into the screaming bleeding thing, etc. If they engaged in hand to hand combat, it would have been out of desperation or to assist in maneuvering a target so they could better transfix the target with something sharp and pointy and it would not have been flashy but quick, brutal, and to the point. The ninja killed folks, usually as quietly as they could get away with while keeping a eye out for making it look near impossible or employ a few theatrics to spread fear and misconception about their abilities and who and what they really were.
As far as Dark Heresy goes, just have a look at the illustration on DH pg 228 and tell me those two aren't Space Ninjas. And on DH pg 120, there's a fine illustration of a couple of Space Ninjas flipping out and killing everyone (there's a better one elsewhere but I can not remember where) -those two Space Ninjas will also soon be wailing away on their awesome air guitars ;-). Though, granted, they are death cultists and not temple assassins, I think the all powerful supper trained can go toe to toe with a space marine assassins of the temples are the apex of Space Ninjitsu.
If the Space Knights get to use fully automatic armour piercing grenade launchers (which the Not Space Knights didn't use), then Space Ninjas can use supper snipper rifles, crazy helmets of psychic doom, mighty morphing power drugs, and copious amounts of brainwashing and alteration to assist them in flipping out and killing everyone. In the end, they still fallow the Space Ninja way of doing things: dirty sneaky tactics, copious amounts of psychological warfare, stealth and misdirection, and stylish black outfits which conceal their identity and help them get all supper ninja stealthy.
