The long answer is that Ice T is an entertainer, and he does whatever he wants so long as it entertains (and makes him money). He went into rapping and then other music because he wanted to express himself (and earn money). He branched out into acting for the same reasons. That Laurence Olivier played a Nazi in Marathon Man and then a Nazi hunter in The Boys From Brazil doesn't make him a hypocrite...it makes (made) him an actor. Ice T himself played a cop in New Jack City the year before Body Count released Cop Killer.
The moral of the story is, people have to stop looking at art and entertainment as propaganda. Even when it has a "message," most of the time it's someone's personal expression. I figure that when Ice T sat down to write Cop Killer, he was trying to share his anger, tell a story, provoke his audience, and sell some records. He was not trying to convince 16 year olds to shoot police officers. Or as this article spells it out:
In the spring of ’92, mere weeks before the King riots, another hip-hop anti-hero matched NWA’s ante. Ice-T, an Army veteran, retired pimp and rap’s self-professed original gangster (his 6 ’N the Mornin’ predates Straight Outta Compton by a year), had taken the unusual step of fronting a heavy metal band, Body Count. Together they released Cop Killer, a thrashing attack on inner-city policing. Bursts of gunfire underscored the song’s venomous lyrics:
Cop killer, better you than me
Cop killer, f--- police brutality
Cop killer, I know your momma’s grievin’ (f--- her!)
Cop killer, but tonight we get even
The American mainstream was horrified. U.S. vice-president Dan Quayle called Cop Killer “obscene.” Sixty members of Congress signed a letter to Warner Bros., Body Count’s record label, slamming the song as “vile” and “despicable.” After an estimated 1,500 U.S. stores pulled the band’s self-titled album from their shelves, Ice-T announced that the song would be removed from all future copies. He refused an apology, though, citing his First Amendment right to free speech. “I’m singing in the first person as a character who is fed up with police brutality. I ain’t never killed no cop. I felt like it a lot of times. But I never did it,” he told reporters. “If you believe that I’m a cop killer, you believe David Bowie is an astronaut.”