In Jurassic Park, Dennis Nedry uses his programming abilities to mess up the whole park while he makes his escape. My question is, why did he turn off the fences? The whole disaster could have been avoided if he had just left those alone.
4At least in the book, I think he only disabled the dilophosaurus fences (the specific cage he was going through). It was only once people were trying to regain control of the system (including rebooting the system) that all the fences were shut off.– Clockwork-MuseApr 16, 2013 at 18:35
Right after Hammond wonders why Nedry would turn off the fences it cuts to this scene:
According to the script the latter sign says:
This Door Cannot Be Opened When Fence is Armed!
Still it wasn't his goal to avoid any disaster, rather on the contrary.
In the end Dennis Nedry was hired by a competiting organization/company for industrial espionage, to steal the embryos. So producing some disaster by turning off the fences could on the one hand help him escape unnoticed or at least unbothered. And on the other hand sabotaging the park could only be in the best interrests of his real employers.
Of course he didn't anticipate that it could also make it much more difficult for him to escape either. Maybe he didn't even anticipate anybody getting seriously injured (or he just didn't care, but "even Nedry knew to keep his fingers off the raptor fences"). From his point of view a little chaos could only work to his advantages (even if he was wrong in the end).