When Jiren’s origin was revealed there were a lot of details that were purposely left out. We were told that a monster came in and destroyed everything that Jiren ever knew and cared about but it was never shown who that monster actually was. It was made very clear that this monster killed Jirens family, his friends, and everything he held dear but once Jiren became stronger this mysterious character disappeared altogether.
Now there is something very odd and peculiar about this backstory as it is told from an outside perspective by Belmond and not by Jiren himself. According to the story, the monster not only killed Jirens family and his friends, but he also slayed his companions and even his master. However, Jiren was the only one that survived this onslaught. Why would such a merciless monster spare Jiren? Was Jiren too strong for him or did he show some final compassion for the young kid?
There are so many fan theories suggesting who the real culprit is but no theory makes more sense than the fact that Jiren was the killer himself. This theory explains everything perfectly. Jiren was so strong from an early age that he couldn’t properly control his power. This is the reason why he had a master that helped him train his powers. However, Jiren could not control his power and eventually he would kill his entire race including his family, friends, and even his master who tried to save him till the end.
This is also explains why the Killer disappeared once Jiren learnt to master his strength. He was the killer all along and once he learned to wield his powers, that monster hidden within him was defeated. Now you can understand why Jiren is always meditating, trying to attain even greater peace within him. That serious look that question why individuals like Goku strive for greater power makes sense once you understand Jiren’s perspective of true power and its consequences.