Killer is Kidd's partner in crime and his RHM, he's exactly like Zoro.
VC is an official role in the crew, Killer's role is that of fighter.
Basically VC and RHM are 2 different things.
A VC is always also the RHM, while a RHM isn't the VC necessarily. In the latter case the role of VC doesn't exist in the crew.
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I loved Killer's reaction