A majority of people have only witnessed Mari killing Yumil. Here is the scene where Zeda kills him instead.
There could have been 3 possible ways to get this scene.
1) I chose to fight the Western Division. Many people don't do this because Ankiseth's unit comes as a beastly addition to your battalion, and he's too badass to pass up, so they choose to make peace with the Western Division just to get Ankiseth. When I chose this option, I noticed the dialouge changes when Yumil was out in the courtyard with Mari and Baldwin. I believe this is the main thing you have to do.
2) I chose paths that would be "shortcuts." For example, I went to Mt Ithaca first and avoided wasting time doing the other scenes. This could have dropped time considerably (average 7 days per battle), and that could matter because in the dialouge with Yumil before Zeda kills him, Mari says he hasn't fallen into the darkness yet, so you essentially cut time to reach Yumil faster so he doesn't succumb.
3) I blitzed through the game. I didn't waste any unnecessary days by doing stuff like timewalking - running around the map racking up birthdays to powerhouse your team with enhancment items. This could matter, again, because you're cutting time. Instead of reaching Yumil when your 50, you reach him when you're 19, That's a good 31 years Yumil has to decide whether to become evil or good. This is probably not acceptable, because you can timewalk before you even meet Baldwin.
I have noticed tho that the boss battle at Latium was King Procus instead of Yumil. After you beat Procus, he runs to Yumil and asks him why he hasn't cast the forbidden spell, then Yumil kills Procus. Then you have to fight Yumil. In the past, I do remember fighting King Procus but not Zeda killing Yumil afterwards. But that was years ago.
That video was done on a legit file. No gameshark or item duping. Just a powerhouse battalion all at level 61 after chapter 12. If you would like to know how I did it then messege me back and I'd gladly explain it. The strategy contains a lot of steps but it's amazing when you can blitz thru every stage, even the semi-hidden Caliber stage.