There are some actions in war that are designated as war crimes, such as targeting civilians, torture, and systematic sexual violence or enslavement. Some of these actions hinge on the definition of "civilian." In an effort to better understand which actions are allowed under international law and which are not, I tried to find an answer to the titular question, but unless the head of government or head of state can be defined as a civilian, I haven't been able to find any evidence that assassination of a head of government or head of state is explicitly disallowed under an international convention.
Does anyone know (or perhaps is anyone much better at Googling than I am):
Is it illegal for a nation at war to assassinate another's head of government/state?