For whatever reason, Zoro stays home and Killer is lost in East Blue, leading to Luffy recruiting him.
How would the story change if Killer was Luffy's right hand man? Would replacing Zoro make Luffy's journey any easier and how would Killer fare in arcs where Zoro was heavily relied on, like in Alabasta, Ennis Lobby or Thriller Bark?
Killer is like a hybrid of Zoro, Nami and Sanji, not better than any of them in their best stats (swordsmanship, intelligence or speed) but very good at all three. I can honestly see Killer being a great boon to Luffy early on in the Straw Hats by being able to put pieces together each arc much better. The only real difference is that Killer is seemingly a lot more like Kidd, bloodthirsty or prone to violence/damage. I can't see Killer enduring the beat down Zoro and Luffy took in Jaya just to prove a point. He might be able to reign Luffy in a bit more though and he seems to be a more active right hand man, organizing alliances between Supernova and all.
Honestly, apart from Zoro being much stronger Post Time Skip, I think Killer would be a pretty great replacement in Paradise.