It was something about the brother stood up against the emperor to save lilliana in the past life.
He had a bunch of siblings, and how I understood, p much all of them betrayed him and tried to take the throne (with maybe the exception of Edgar's parents? Or then they also betrayed him, or were part of a plot where he was betrayed - but bc one of Edgar's parents is a sibling of Leopold, I think that relates to how Edgar is still cared for. How I remember/understood it.) + the newest ch (37) also has Leopold mention "like your siblings and uncles", meaning Marius was not betrayed by only his siblings (whom he cared for), but his other "family" as well.
Basically, it is a fear of his to be betrayed again by someone he loved. He could also have thoughts that those people betrayed him precisely because he cared for them - it hasn't been said directly, but as someone who has experience trauma, I kinda guess this could be a reason/thing that exists as well. Yes, it is selfish of him to do that to a child, and to be so cold and indifferent too.
As for a flashback/scene where we saw a blond guy holding a sword against Marius, I don't think it was explicitly said it was referencing the past? At the very least, I interpreted it to be a scene of the future Liliana(?)/MC has seen, where Marius' indifference/coldness caused Edgar to turn bitter and do the exact thing Marius was afraid he'd do. (Again, his own fault) but like I said, this was just how I intrepreted that scene, and I could be wrong on it. ╮(￣▽￣)╭
Hope that helps!
OHHH THANK YOU SO MUCH!!!!
UHHH WHATS WITH THAT LOOK
can u guys explain why the emperor hates his brother? (i think there was an explanation why but im not sure cause i forgot that’s why im asking rn teehee) thank u in advance (づ￣ ³￣)づ