Just to be clear, Saturn wanted Kizaru to
ignore Sentomaru on the lower level and head straight to the dome on the upper level and bypass it (where Luffy and everyone else is)
But Kizaru
refused and wanted to fight Sentomaru first, because:
1- he's paying respect to his determination and duty
2- if he doesnt take him out fast, then the Pacifistas and Sea Beasts wont receive orders from anyone = therefore the Marine fleet is safe from them
just thought I'd throw that out there.