Kizaru could have killed Sentoumaru, he clearly held back against him, Bonney, and Kuma, he didn't really try to get up and fight again after Luffy, "laid him out", even Saturn gave him a pass which people ignore for some reason. Koby at Marineford had the role of being part victim but also saving Luffys life. Now Koby is setup to directly be in the way of Luffy's dream knowing he would have to kill him and be strong enough to do so. He doesn't trust Luffy to lead the world and rightfully so, 99% of the verse would abuse the OP.