3. Dennis Nedry - Jurassic Park
Dennis Nedry might be one of the dumbest smart people ever; a computer programmer for Jurassic Park, he is in fact a double agent, accepting a payoff from one of InGen's rivals in order to sneak them some dinosaur embryo samples. Under the pretense of performing a system reboot, Nedry de-activates the security protocols, allowing him access to the embryo storage room, but this means that all of the electrified fences are
also de-activated, allowing the T-Rex to run rampant around the island, endangering the lives of all the human survivors. If Nedry's patent lack of foresight is one thing, it gets worse. Nedry is driving to meet his contact, when he crashes his jeep, and in trying to fix it, is attacked by a Dilophosaurus, which promptly kills him. At least Nedry ejected himself from the gene pool through his stupidity, I suppose.