Here's my theory on the matter. Bear in mind I haven't read the LN.
Before time travel, Reid used to be the Sage as that person implies he'd become without intervention after becoming a slave.
On the other hand, the one who became the enemy of the world, the original Hero, was Elria. Her power was natural, and without Reid as a balance, she would have been peerless and lonely. As the Sage, Reid was likely the leader of the coalition against her but also one of the few who understood her. But, he was still weak. In her "hopelessness," she entrusted the time traveler with her power to pass down to Reid and died. I imagine this time traveler is strong in his own right since he needed to waste none of Elria's power so that Reid and Elria could become equals.