If killer was gone or dead, he would lose his trusted and be more distant, and have less Confidence and Support to his teammates, and wouldn't let others close to him.
In a fight his back would be open and could be shot down by dream from behind, because all the others would be too far away from nightmare, fighting close up to the others people. nightmare uses his tentacles and his tentacles launch forward to attack so his back is always open because the tentacles are attacking at the front. And that's why killer is there to protect him and fight close to him, also taking some hit for him.
If killer was not there...
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This is a headcannon for why killer is always known for fighting around nightmare, Usually back to back-
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