You can say anything you want in an ad, subject to whatever profanity restrictions apply. Calling someone a "bug assassin" may be perceived by some as a negative, but not for the reasons you think. In gaming culture, the idea of assassin (or its Japanese equivalent, ninja) is something people aspire to. Look at the plethora of ninja games, or the Assassins Creed series if you don't believe me.
No, the problem I foresee is with the word bug. That instantly signals that the person you hire will be maintaining buggy, possibly hopelessly substandard code. As a programmer, there is nothing I would like to do less than maintain someone else's crappy code. Certainly there are people out there who don't mind that kind of work (especially those who are currently unemployed), but the job is not one many people would interview for unless that were the only choice. YMMV.