Be careful what you cast... it could bite you in the ass. Then again, LoR isn't just about casting either...
There are no ten commandments in witchcraft and contrary to popular fluff the rede is not a commandment, it's a pretty poem. Most trad wiccans know this and most witches are not wiccan so it's a moot point. The LoR is universal though, you see it in one form or another in all religions. You reap what you sow. If you don't really understand that old saying, it has to do with planting and harvesting, you harvest what you plant, you get what you give.
So when you go to cast a spell for vengeance what exactly is it that you're willing to sacrifice to get temporary satisfaction? It could be anything, even your own life... or that of someone you love dearly. Is it worth it? Are you willing to pay the coin?