Rayleigh is obviously weaker, but it was a 1v1, Kizaru didn't get any external help and was cornering Ray on his own.
Kizaru didn't defeat Marco just like Aokiji wouldn't, plot convenience for Marco to lead the Remnants of the WBP.
And again, Akainu had Kuzan at his mercy, he could had kill Kuzan, but he chose not to, Kuzan is only alive because Akainu wished so, Kuzan lost a leg.