Jason Griffin is charged with manslaughter, unlawful possession of a firearm and felon in possession of a gun for the deadly shooting.
Investigators say Griffin was at a party in the 300 block of West 23rd Street.
At some point during the party, the victim put on a bulletproof vest.
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Police say Griffin took out his gun and shot the victim in the chest.
The victim was killed.
Griffin left the party before returning to talk with police.
His girlfriend told KTRK-TV he didn't mean to kill his friend. She said Griffin didn't even know his gun was loaded.
"He goes 'shoot me.' Well, he didn't think there was nothing in it, and he did. And the dude went '@##$ that hurt' and then he dropped. And then Jason ripped the vest off of him and tried to help him,'" said Mary Warstler.
Warstler said Griffin was suicidal after the shooting and had to be treated for seizures before he was arrested.
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Homicide Sgt Mark Holbrook said there was a small party.
"The victim put on a bullet proof vest and another man who has a gun winds up shooting him," he said. "It's unusual, but these things happen sometimes."
Mary Warstler wasn't at the party, but she's the longtime girlfriend of the man police are talking to.
"He went there to sell his friend a flak jacket," she said she was told by her boyfriend. "It's so bad because he didn't know the gun was loaded." She identifies her boyfriend as 39-year-old Jason Griffin.
She said he told her that he went to sell his friend the bulletproof vest. According to what she said Griffin told her, "he even asked, 'Brother, do you want me to cinch that up for you,' but they were playing this stupid game going around."