When playing around in my world, I released ravengers on iron golems. After the fight, there were a few iron golems left. I used the /kill @e[type=iron_golem] command, and the screen reported that the command had worked. The Iron Golems looked like they had taken damage, but they were still alive. I spawned in a chicken and did the same, which gave me the same result. After some more testing, the command kills the player and armorstands, but not mobs.