When you prove victorious against a warring army in Mount and Blade II: Bannerlord, you obtain a handful of their forces following the victory, along with any loot they were carrying. For those who do battle against larger factions and face off against a leader of that group, you may obtain a lord as a prisoner.
Rather than ransom them or keep them as a prisoner, you have the option to execute them. The lords and faction leaders are the only members of the game you can freely execute. You can’t do this with the other prisoners of the game. If you want to remove some of your prisoners, you can choose to release them, add them to your army, or sell them to one of the large settlements you visit.
When you wish to execute one of the lords you have in your prisoner’s tab, jump over to it in the party menu, and highlight the one you want to execute. You should have the available option, which is the skull and bones icon when you choose the lord. You receive a brief warning right before you execute them. The game warns you that doing so will have negative impacts on your reputation, decreasing your relations with any faction members associated with that person. Doing so kills the lord from the game, and you will no longer see them during the campaign.
Decide if you believe it is not worth continuing positive relations with any faction members apart of that group. However, if you eliminate everyone in a faction, everything they owned becomes apart of your clan. For those who wish to obtain everything from a rival faction, you will find some positives in performing this action, but you need to go through with every member of that faction. You can’t let one get away from you.