I think Oda might actually be trying to make Shamrock a sympathetic figure
He's the child who wasn't saved, never had the chance to be free, and was indoctrinated in the Celestial Dragon way of life.
He also seems to have been close to Harald at one point, he treated Loki with a strange amount of respect, and he doesn't seem to resent Shanks that much.
I believe Shamrock betraying Imu and Garling might be an inevitability