He hates the Blades. Through trickery and deceit, the Regulators convinced him they are responsible for the death of his son, Jason, who had lived outside Adytum. Truthfully, his son died at the hands of the Regulators, who wanted a reason to get rid of the Blades for good.
Jon Zimmerman was executed by a Regulator guard during the Blades' attack on Adytum in 2162. After hearing Razor's holotape recording of the Regulators planning to execute Jason, he was shot for discovering the truth behind the so called 'peacekeepers'.
Kill Razor: When first spoken to, Zimmerman will explain the recent occurrence of his son's death. Put simply, he offers the player 2,000 caps to execute Razor, having been tricked into thinking the Blades are at fault.
Although extremely difficult, it is possible to prevent Zimmerman's death at the hands of the Regulators:
Prior to initiating the conversation with Zimmerman to expose the Regulators, position the PC between the nearest Regulator and Zimmerman. As soon as the conversation ends, enter combat mode. The timing is critical; wait too long and the Regulator will get off the first shot, but acting too quickly can set all of Adytum as being hostile. Tactically, the idea is to use the PC as a combination mobile shield and firebase. Once the Regulators in the room have been eliminated, the PC should, for best results, station themselves in the exit door, plugging the way out and trapping Zimmerman inside. As the Regulators hurry to the building, they can be picked off as they round the corners.
If the battle manages to get to the street, keeping Zimmerman alive becomes much more difficult, but still possible. Keeping an eye on all lines of fire between Zimmerman and the Regulators becomes the primary issue. Station the PC between a ranged-armed Regulator and Zimmerman, and try to gun down first other ranged-armed Regulators, then any melee Regulators trying to close on Zimmerman. With a little luck — and a lot of ammunition — the battle is winnable. However, since the game doesn't really expect Zimmerman to survive, his reactions following the battle are a little quirky.
If Zimmerman is alive after the battle, using the method above, it is possible that he will become hostile and attack the PC. The rest of the citizens are now hostile towards Zimmerman. Killing him will not make the rest of the citizens hostile.