A novice hit woman accidentally kills her first employer and now must take out an avenging mob when they threaten to kill her family and relatives.

I like the concept and the logline too. I do wonder if a little 'color' can be added. The structure of the logline is sound so it's hard to tinker with. Just an example, what if it led with "An inexperienced, [ADJECTIVE] hit woman accidentally kills..." The adjective is whatever primarily describes the character.

You could cut out additional verbage by using "an avenging mob hellbent on killing her and her family." Using the word 'threaten' seems unnecessary. Is the avenging mob a mafia family? If so, what nationality? You could say "Italian mob" or "Irish mob" or whatever it is.

I second the recommendation to take out 'and relatives', its unnecessary words that could be used for something else. I am guessing this is a black comedy-type movie?

Okay sharkeatingman...i will heed your warning. I do like the shorter version without giving away too much about the story.

Change it back- immediately! I can't sit idly by and watch someone destroy what was once close to perfect (in my opinion, of course). If this great concept is ignored because of a bad logline, I'll shoot you myself! ;)