Elbaph showed us that Imu from very far distance can take over the body of Gunko, one loyal Holy Knight.
Now at the flashback we saw how he took over Saturn as well. One loyal Gorosei.
But then why he didn't took over none of the 5 Gorosei back at Egghead?
Of course the most obvious answer would be that Oda didn't thought about such power back then but that alone can't be enough. We have to figure a reason inside the world of One Piece.
For starters I don't even think that right now Imu was necessary to show up at Elbaph. He could have asked Shamrock to return there. They could have summoned Gorosei (I think). And the same could have been done at God Valley. So I don't think Imu only shows up in situations where the rest of Holy Knights or Gorosei can't handle.
We also know that Saturn got killed after the Egghead incident. Is not like the Gorosei have failed for the first time so I don't think this would be enough reason even for a tyrant like Imu to kill him.
The best explanation I came up with was that Imu needs a loyal servant to take over his body.
Back at God Valley it was Saturn.
Now at Elbaph it was Gunko.
Back at Egghead Imu tried to take over Saturn but couldn't. This means that Saturn right there wasn't as loyal as he was back at God Valley and that's why Imu killed him afterwards.
After all not only Saturn was there but all 5 Gorosei failed at that mission. So no reason only for Saturn to be killed.
Now at the flashback we saw how he took over Saturn as well. One loyal Gorosei.
But then why he didn't took over none of the 5 Gorosei back at Egghead?
Of course the most obvious answer would be that Oda didn't thought about such power back then but that alone can't be enough. We have to figure a reason inside the world of One Piece.
For starters I don't even think that right now Imu was necessary to show up at Elbaph. He could have asked Shamrock to return there. They could have summoned Gorosei (I think). And the same could have been done at God Valley. So I don't think Imu only shows up in situations where the rest of Holy Knights or Gorosei can't handle.
We also know that Saturn got killed after the Egghead incident. Is not like the Gorosei have failed for the first time so I don't think this would be enough reason even for a tyrant like Imu to kill him.
The best explanation I came up with was that Imu needs a loyal servant to take over his body.
Back at God Valley it was Saturn.
Now at Elbaph it was Gunko.
Back at Egghead Imu tried to take over Saturn but couldn't. This means that Saturn right there wasn't as loyal as he was back at God Valley and that's why Imu killed him afterwards.
After all not only Saturn was there but all 5 Gorosei failed at that mission. So no reason only for Saturn to be killed.