Shanks killed Cobra. Cobra and Shanks meet with the 5 Elders. There’s a possible silhouette of Shanks as one of the 9 Holy Knights. The shocking thing that Sabo saw wasn’t Imu on the throne, it was Shanks acting as a Holy Knight and killing Cobra.
Remember, right after Shanks meets with the 5 Elders, Imu looks at Vivi’s photo with interest right before being asked “The light that needs to be extinguished from history… Have you decided who it should be yet? If you have… Please tell us their name!!”.
The light to “extinguish” is the Alabasta kingdom, a shining example of a thriving kingdom where the people love their royalty – the complete opposite of the Celestial Dragons. We also know the 5 Elders have some general distaste for the Nefatari because they refer to them as “traitors”. The 5 Elders also hope that Cobra “doesn’t make things complicated”.
So, we know there is some type of conflict between them, and the Elders might have what they deem to be reasonable cause to have him assassinated. Shanks met with the 5 Elders to talk about “a pirate,” and “Given [his] standing…[he] should have nothing to do with politics”. We know that Shanks was found at God Valley, but he doesn’t resemble any of the Celestial Dragons. The reference to “his standing” may imply that there is something more to him than being a Yonko. When Croba meets with the 5 Elders, he sends Chaka and Pell away claiming that “it would be tedious” for them to “wait outside” even though the meeting “won’t take long.” Sending his bodyguards away because waiting outside for a short time is tedious just doesn’t make sense. Croba probably did figure something out, confronted the 5 Elders, and then the 5 Elders have the closest Holy Knight assassinate Croba – but they can’t take the blame, so they pass it off to Sabo who happened to be in the right place at the wrong time. Shanks killed Cobra!
*Theory by ScruffyLookin-