i can see, Luffy attacking Blackbeard over defeating Law later on.
Shanks wiped out a portion of the straw hat grand fleet, and Kid's entire crew.
Those pirates aren't nakama Luffy will not attack Shanks over that, will he actually?
Blackbeard might've captured Garp, but Law's defeat might be the driving force for Luffy vs Blackbeard.
That means Luffy and Akainu share a common enemy. Luffy shouldn't fight the emperors, but Blackbeard is most likely his villain.
This is why we are getting a flashback on Xebec, it's about Blackbeard's famous dad, and why BB got revenge on Garp.