maybe Blackbeard is here to recruit the evil sword sub-commander of prison? he'll gain a worthy swordsman (he lacks a guy to fight against Zoro, as was mentioned in the anime discussion)

my hunch is that he wants to put on a mirror show to ace's execution, he's planning to kill a highranking marine in the same time. I can't figure out a reason to do so, but that's my gutt feeling.