If you run the /kill command in a function, the entities you've killed are not removed from the world until the next tick. This means you get unexpected behavior with code like the following:

kill @e[type=armor_stand,tag=Marker]
execute if entity @e[type=armor_stand,tag=Marker] run say Found!

You'd expect it to not say anything because all the Marker stands are dead, but it will still output because the stands aren't removed from the world until the next tick.