My guess is that the three people killed and the start (probably isekaid) including the MC are the reincarnations of the three great magicians of the past. And the dude who killed all three of them is the hero’s reincarnation. I bet it’s like spirit circle where they are bound to be reincarnated together over and over again. No that I think about it, the demise curse from Legend of Zelda is also the same concept. With Zelda, Link and Ganon doomed time be reincarnated over and over again and cause conflict.