Why He Should Be Good At His Job:
He's a cop and he's robot. Period. There is no fudge factor, no gray area. He is literally programmed to do things by the book.
Why He Sucked At It:
Let's analyze RoboCop's very first case step-by-step.
A perpetrator wielding a machine gun is robbing a convenience store. RoboCop arrives, is promptly shot 30 times and shrugs off the bullets like NERF darts, and proceeds to disable the crook's firearm by bending its muzzle. Now that the criminal is disarmed it's time to slap on the handcuffs and...
BOOM! ROBOCOP CLOTHESLINES THE SUNUVABITCH INTO A FREEZER! BLAM! CRIPPLETASTIC! WOMP!
Our cyborg hero then leaves the store... without arresting the guy. OK, so it was his first day on the job--maybe he had the first time jitters. Perhaps he'll show a little more restraint with his next case ...
KAPOW! HE CASTRATES THE BASTARD WITH GEOMETRY!
Again, RoboCop makes no real effort to apprehend the crooks. It goes on like this for the next 90 minutes. All in all, RoboCop arrests only one criminal during the entire movie. The rest of his time is spent alternately maiming people (three guys) or blowing them away (he racks up a body count of 16).
This is pretty confusing as RoboCop is, in fact, hardwired to be a good guy and has the free will of a VCR. Who the hell programmed him to be a cop? The T-1000?