You started killing to make them stop what they were doing.
If the guy comes after you, he might start killing your patients, too.
I don't think you and your men are ready to start killing one another.
He did not need these men to start killing each other.
I just got in the door and started killing them.
Then you go onto the playing field or court and start killing them.
"After all, we could have started killing each other a long time ago."
But I think he started killing her that very day.
"But then they might get in a time machine and start killing us."
"Why did he stop for a while, then start killing again?"