Luffy? Pretty much definitely.

Zoro? Hell yes; even if we disregard everything after he joined the crew, he was a pirate hunter. He uses swords. He's killed plenty.

Sanji? Hard to say. Never directly on-screen, but I'm putting this as 'plausible', scaling up to 'most likely'.

Chopper? (*cough*Hiluluk...please don't kill meit'sthetruthtechnically). Despite that, he's a doctor. He doesn't kill, he saves lives, but I'm sure he'd be willing to for the right reasons.

Nami? Arlong Park. 'Nough said...

Usopp? Nah. It would probably tear the poor guy apart. I can't recall him ever killing anyone.

Robin? You better bet your kiester she has.

Franky? He's more than likely a killer, especially in his past with the Franky Family and the fact he turned his body into a freaking weapon of mass destruction.

Brook has absolutely positive killed lots of people. Even if we ignored his career in his kingdom's military, that final fight that poisoned/killed his old crew was bloody on both ends. He undoubtedly killed lots of the attackers, being the captain. Also, do zombies count? 'Cause then he's a mass murderer.

As for all the fighting we see in the series, we see a lot of KOs, especially from Luffy and Sanji's fighting (blunt force fighting). I imagine plenty of the slashed-up people that Zoro and Brook fight bleed to death. It's just that it never gets focused on, because the series isn't meant to dwell on the morality of killing throngs of low-level underlings--it just kind of sits in the background, a general acceptance that death happens. The Straw Hats kill people out of self defense, but undoubtedly try to resort to less lethal methods before going all out.