Theory: The motive videos work in the same way as the Flashback Lights.
Possible evidence
Kirumi stated that she remembered being prime minister just from watching her own motive video. So once she watched it herself, she believed that what was on it was true and part of her backstory.
Kaito stated in his 4th FTE that the current prime minister was one of his previous sidekicks. Now you would think that he’s talking about Kirumi, but it should be noted that you can get Kaito’s 4th FTE in Chapter 2, and when he saw Kirumi’s motive video, like everyone else, he didn’t want to believe it was true. So since the beginning, Kaito couldn’t have known Kirumi was the current prime minister, so he could’ve been talking about someone else.
After Kirumi’s execution, Angie was the one to suggest the motive videos being similar to the Flashback Lights, where if the specific person was watching their own motive video, they’ll believe that what’s on it is true.
Kokichi also got his own motive video and it stated that his organization DICE did “petty non-violent crimes and harmless pranks”. So if Kokichi saw it, he’d be smart enough to not actually commit murder.
Even though Ryoma's motive video says he has no one important to him, in his FTEs with Kaede, he said that he had a cat that he cared about. Now think back to Ultra Despair Girls, it's revealed that Celestia and Toko both had animals in their motive videos, so how come Ryoma's cat wasn't in his motive video?
The only thing that would get in the way of this theory is Maki’s motive video, which most likely reveals her secret about being the Ultimate Assassin and Ryoma was the one to get it. It’s more likely that since the beginning, Maki was already the Ultimate Assassin and simply used Ultimate Child Caregiver to hide that truth.
