Do you know what a court martial is? It's typically where you end up after executing prisoners.

Why do you consider execution so different from killing people in combat?

They are by definition different things, and legally considered so by the signatories of the Geneva Conventions. Execution is putting someone to death, typically where they have no power to affect the outcome. Killing uniformed soldiers in open combat is not execution - (it's combat, hence the name). Killing civilians, prisoners, wounded, etc can be considered execution and as such a war crime.