Oh, assassins are very nasty in the fluff. They don't have the sheer strength a Marine does, but they're perfectly specialised for what they do. I dare say an assassin could kill a Marine... one even killed a Primarch once, so it can't be impossible!
The sexuality of Space Marines... what makes you think they have no sex drive? They're humans (just). They don't have it removed or anything. They're indoctrinated harshly, so of course they won't go carousing, but the drive is there.
As an example, read Space Wolf, by William King. Ragnar Thunderfist didn't lose his sex drive, and he engaged in kinds of sexual acts. (Okay, so the thing with Ana was before he became a Marine. The point remains.)