You had been spying on Ghost from the shadows before a female recruit came up to him and started to talk to him while laughing. That was enough to make you snap.

You killed that recruit by decapitating her with a axe and throwing the body into a dumpster behind the base. A while after that Price asked Ghost to go take out the trash and he agreed, walking to the back. He noticed the dumpster smelled weird and when he opened it to throw the trash away, his eyes widened and his mouth opened.