strange how the brain controls the brain 🧠

The Death Knight's nature as an Alter means that he was created to take all the pain and trauma that Emile couldn't-which we know was BAD from the little that's been revealed. And Three Hopes confirmed (by way of his Supports with Catherine in that game) that he's searching for someone "worthy" in combat so that said person can kill him. (Like, both Jeritza and the Death Knight seem disappointed that Catherine didn't kill him in their fight.)

I think that, after only knowing pain and bloodshed his entire life, the Death Knight thinks that the only way "out" is through violence.

I completely agree with this. In Hopes especially, the Death Knight seems to attack people out of a desire to get himself harmed or killed. I looked into Jeritza’s timeline a bit more, and since his father’s murder occurred in his mid-to-late teens, it’s likely that Emile’s Death Knight alter existed years before this event, since DID manifests in the preteen years. This means there was a build-up of trauma that led up to murder of his father.