actually, I think Blackbeard is here for something else, his end goal is to become the pirate king, so while he should want to eliminate the competition, he might want to even the scales again, espcially since he knows that the goverment is planning on killing Ace, whitebeard, and got the top rookies already.

my guess is that he's going to release Jinbei, use him as a bait to kill whitebeard, but meanwhile, he'll also release luffy, croc and ivanna.

I don't see any reason thay blackbeard might want to kill Luffy, he already won his fame and position, and he seems to have taken a liking to luffy, there's hardly anything for him to gain here if he simply wants to hunt pirates.