You got it all mixed up FFL. Check the link the asker gave. Kaidou might assume that WB would fall in the hands of the marines so he might wanted to take the honor of killing WB instead. Shanks intercepted Kaidou so he won't interfere with WB plans.
Originally Posted by FireFistLuffy
Remenber WB and Shanks talked before the war. Shanks knew he can't convince WB to stop Ace from pursuing BB. He knew Ace would fall and a war is inevitable. After their clash of swords, Shanks might just offered his help it came down to it which it did.
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