Forward-canon: the murdered shipmate is canonized as Thatch, the 4th Division Commander, murdered by Blackbeard to steal the Yami Yami no Mi (Darkness-Darkness Fruit). The on-page Marineford-war pivot is the Ace-pursues-Blackbeard / Blackbeard-captures-Ace / Whitebeard-comes-to-rescue chain.
Why this matters
Ch. 159 line is the structural pre-load for the Marineford-war causation chain. The single phrase killed a shipmate and jumped ship is doing work across 400+ chapters: it establishes (1) that Blackbeard is a known Whitebeard-defector; (2) that killing-a-crewmate is the worst crime in the pirate-world; (3) that Ace is structurally obligated to pursue. The crime-without-victim-name beat is Oda’s signature deferred-name reveal — the victim-name (Thatch) and the motive (the Yami Yami no Mi) are both forward-canon. The line is the load-bearing single-sentence pre-load in the chapter.