Comment by Galactic Navy Officer on Cheat Skill "Shisha Sosei" ga Kakusei Shite, Inishie no Maougun wo Fukkatsu Sasete Shimaimashita - Chapter 31

@Galactic Navy Officer they're both powerful demon lords, one's just more so, and the other now has the power to resurrect themself should they somehow harness his power

@Yes Licht's power only activates unconsciously when he himself dies. Otherwise, he has to consciously invoke it. 

So psycho demon lord has to 1) try to manipulate or force Licht to use it 2) when Licht is not going to want to and 3) can't be threatened with death to scare him.

So it's torture or manipulation. And pyscho not only has to override Licht not liking him, he has to override Licht's loyalty to the Demon King.