Zim was sitting in his living room when it all started. He had been working in his lab, making various upgrades and repairs to the Voot Cruiser when he got frustrated with one part that wouldn't do what it was supposed to. Finally he gave up and stormed upstairs.
He resorted to watching T.V. because he couldn't find anything else to do. He wasn't really paying any attention to the show, for he had it on mute. He didn't even know what he was watching, something to do with baking a cake.
It was rather quiet seeing as Gir was nowhere to be found. Wherever the crazy robot had wandered off to for the time being, he was someone else's problem for now. Zim had briefly thought about cleaning, something he had failed to do lately because he was so busy with the modifications on the Voot Cruiser. There were various clothes, plates, trash and even weapons scattered all over the place.
Dib had suddenly burst through the front door with Gir placed under his arm, disturbing Zim's, for once, peaceful thoughts.
"Zim! Take your stupid dog thing and keep him away from my house! And tell him to stop messing up my kitchen!" Dib had yelled throwing Gir into the house.
Maybe if Zim hadn't been in such a bad mood that day, he would have been able to remain calm. He wouldn't have snapped like he did and start to scream at Dib. Maybe his sudden outburst wouldn't have startled Gir so much. And maybe, just maybe, Zim would have been able to avoid the following events.
But Zim WAS in a bad mood that day, and a very bad mood at that. Zim DID snap at Dib and had started screaming at the top of his lungs. And Zim DID startle Gir into picking up a random object, which just so happened to be a gun, and throwing it at the wall. The gun had went off and the bullet hit Zim straight in the chest.
All of these events brought Zim to where he was now, standing in the middle of his living room staring down at his own limp body lying motionless in front of the couch. Zim remembered the pain of the bullet hitting him in the chest, and he remembered the shock of being shot by his own S.I.R. unit. Figures, the only time Gir actually acts like he's supposed to and goes into defensive mode, he ends up shooting his own master. But suddenly all the pain was gone and Zim was left with the only conclusion he could think of. Zim was dead, and now looking down at his own body, as a ghost.