A man ties two people together and says:
I'll free you from the bad blood
that shackles you to each other. Unites them?
Keeps them chained together (in a figurative sense). "unite" is usually a positive word: if you're "united" then it's a good thing. "shackle" is a negative word: if you're "shackled" then it's a bad thing.

It's a little hard to see how tying two people together can free them from a schackle. The only thing I can imagine is that forcing them to be in close proximity, together in the same unpleasant situation, will help them to resolve their differences. Dunno.
Thanks, Mr Wordy!