Zoro is probably mostly just mad/irritated at Sanji. he said earlier that the other crew members on Zou couldn't possibly be dead because they had Sanji. now, instead, it's Sanji who is the issue
Zoro has a love/hate relationship with Sanji and secretly loves him to bits, but too manly to express it in a normal way. If Luffy said let's keep adventuring and come back to this issue later he would have more than raised eyebrows. Acting tough is part of his character.
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@uniaka:
The thing about Killer is that he seems made for Zoro, both of them as the only non-captain supernovas, sword fighters( I guess his blades can be called swords). If he was Sanji's brother, I would expect him to rival Sanji.
Brothers don't need to rival one another, and if Killer is indeed Sanji's older brother this will be Zoro's one and only chance to beat down someone that resembles Sanji without holding back. It would be hilarious.