-Fred Hampton was a black activist from Chicago -- an extraordinary speaker, youth organizer for the NAACP.
-He joined the Black Panthers and shone so brightly that he was made chair of the Chicago chapter when he was only 20.
-He founded the Rainbow Coalition, which brought together Black and Latino activists and radical anti-poverty Catholics. He forged an alliance among major Chicago street gangs to help them make peace and work for social change.
-In 1967, when he was just 19, Hampton was identified by the FBI as a “radical threat.” The FBI tried to subvert his activities in Chicago, sowing disinformation to get the groups he’d drawn together to distrust each other, and getting an FBI plant next to him as a bodyguard.
-(This is part of an illegal FBI program called COINTELPRO, which aimed to paint black civil rights activists (among others) as violent and threatening. If you’ve only seen pictures of the Black Panthers as armed and dangerous revolutionaries, and never heard of their children’s breakfast program, their community health clinics, or their “copwatch” patrols, this is why. It’s because COINTELPRO was a highly successful work of political propaganda.)
-On December 3, 1969, Hampton taught a political education course at a local church, and then several Panthers gathered at his apartment for a late dinner. One of them was the FBI plant bodyguard, who drugged Hampton.
-At 4:45 AM on December 4, a squad of Chicago Police officers and FBI agents with a warrant to search for weapons stormed the apartment. Investigations later showed they fired between 90 and 99 times. The Panther on security detail, Mark Clark, was holding a shotgun. He was shot, and the gun went off into the ceiling. This was the only shot fired by the Panthers.
-Fred Hampton, in another room, didn’t awaken. He was shot in his bed. Twice, in the head, at point-blank range. He was 21.
-Four weeks after witnessing Hampton's death, his finance Deborah Johnson gave birth to their son, Fred Hampton Jr. That’s him in the photograph, visiting the grave of a father who died before he was born. A resting place riddled with bullets.

Nobody actually knows who shot the headstone, it was discovered damaged in 2016:
As far as I can tell the notion that it's a police tradition seems to originate from a possibly sarcastic post on r/stupidpol on that other site.
But he was definitely assassinated by the police and the FBI, which is probably much worse.
I guess the way the idea police tradition theory has taken hold so well is because it's so plausible.
Eh, I think at least for the past few decades it isn't actually too plausible.
Any known discharge of a firearm by police requires some sort of review. While they may be super lax and claim all sorts of false rationalizations for shooting a living person, I think even they would be unable to come up with a rationalization for discharging at a headstone in a public area.
Now if it is "take a whack at the headstone with a crowbar", I could believe that, but I don't think they'd risk shooting a headstone out of the blue.
I think "known" is the operative keyword here. If a police revolver goes off without a citizen to hear it, did it still make a sound?
Revolvers aren't exactly known for being stealth... It's just too much risk to desecrate a grave of someone who was dead before most of those officers were even born that most folks have never even heard of. At this point even the oldest on the force would be a generation removed from any officers that were working in 1969.