He has to at least kill the head of Systems Corp.; he said that at the beginning of Thing-Thing 4: "The only way to end this is to go to the top, and eliminate whoever is in control of all this insanity...I'll have revenge on whoever created me. Nothing is going to stop me." by the sheer power of the plot, a dues ex machina of some sort will resurrect him. I'm thinking, it seems like the gas in that last level keeps you from healing. Maybe they turned it off after they thought he was dead (which would make sense; I mean, why waste precious anti-healing gas on a dead guy?). Perhaps he isn't actually dead (the confirmation comes from the head of Systems Corp., Who is miles away and viewing the whole thing remotely), maybe he realized that the gas was suppressing his healing powers and faked death so they would stop shooting at him)At the end the guy said,"take his body to genetics and have them map his genes," if there's a body, he may be somewhat intact, at least.
At the end of the Game the head guy of system corps says that pro.154 is going to die and how they were going to rebuild him with his temper and stuff gone so the screen goes blank and these guys come in and they keep coming until you die and then you have beat the game. So im really hopping that they will come out with thing thing 5.
