Still, it's a bit better than having Ollie and Slade fight over the affection of Shado on the island.
To Slade in choosing Sara Ollie chose to save a stranger over a comrade.
The three of them have been through a lot together on the island, forming a bond that should be stronger than Ollie's infatuation with his girlfriends sister. In a way, he knows Shado and Slade better then he knew Sara at the time. And yet he let Shado die.
When I look at it like this, it's not that bad. Also Slades ghost saying "You're not a friend, you're not a brother" kinda implies this. I'm sure there will be more to it as we learn more in the flashbacks.