Interestingly enough, Oda doesn't 'mind' the fact that his characters might actually kill someone but he does have very specific ideas on which members of the crew will or won't kill. There are certain crew members who -by virtue of that policy- cannot and will never kill anyone.
Not like it matters too much considering his policy on death in general.
Do you say that based on your reading of the series only or are you also aware of some remark made by Oda?
Also, from my point of view that list would put Zoro, Robin, Franky and Jimbe on the Killers side.
Zoro and Robin are kind of obvious but I see Franky and Jimbe on that team too since they give a vibe of Adult with a Harsh Life and Serious Problems.
Luffy, being the hero (and using Oda's comments on it), doesn't pull punches when fighting but ends up only beating his opponents dream. Chopper is the innocent one, the biggest opportunity he had to kill someone was when he couldn't control his monster form. Nami should just stay away of such violence due to her backstory, it's calming to think she would never be capable of such cruelty as Arlong. And I'm guessing I'd put Usopp on the no-killing list too, he seems to be the only sniper in the world capable of ending fights without a fatal headshot and that's even harder, imo.
Don't know where to put Sanji and Brook though. Both seem to be more deeply rooted on their "life-giving" side, with cooking and music, but Sanji has a Do Whatever it Takes attitude sometimes and Brook has, like Jimbe and Franky, led a harsh life.