I was at battle with Aserai and their lord Tais died during the battle. I didn't kill him or I didn't execute him after battle but I got relations penalty with 4 clans, including Rhaegea's clan Pethros. (which I'm her vassal) I mean people dies in wars, it isn't my problem if he is an idiot and jumps in to my soldiers. I'm assuming this as a bug, not a new feature. I hope. If this is a new feature, I'm going to execute every lord that I captured and not gonna tire the game up.