Too long for a comment, so... Comments have indicated that killing is a noun, or rather a gerund. Thus the verb form occidere might not be appropriate. The noun form is occisio. You have also indicated that the meaning is "the killing is done," a passive form.
You already have a translation using a different word order, but perhaps for a tattoo it might be written
in nomine signi X occisio facitur
as (with a bit of artistry) that would allow a symmetrical design with the symbol in the middle.
However, others are more well-versed than me in the Classics, and it's at least conceivable if not likely that the phrase has already appeared in classical Latin. As mentioned in comments, kill has many synonyms like slaughter, murder, slay, execute, butcher, put down, put an end to... and one of those might be a better fit — or, maybe, indicate that an existing classical use might not fit.