When a US citizen takes up arms against the US and they are killed in the field, that is one thing.
But when the government goes out with the specific objective of executing a US citizen without due process that is the worst type of violation of US laws by the government and the perpetrators should be tried for murder.
This is not justifiable homicide, by any means. It's 1st degree murder and conspiracy to commit 1st degree murder that we are looking at here.