*Repeatedly throws molotovs at Denise until she drops then I take her weapon"
Now that I killed both black girlfriends, I should point out it wasn't racially motivated. Neither a fan of cheaters nor children.
GTA.Wikia (editable by anyone, by the way) says this: "It is possible that Denise is a prostitute, because she was seen with Los Santos Vagos, and she gives CJ a Pimp suit once he reaches 100% progress with her. It is also suggested by her car which is a "Hustler".
The pimp suit is purple, so it's possible she killed a Ballas member and stole it off his back.
Did any of you know that killing the "Vagos" in Madd Doggs mansion and the "Vagos" outside the East Los Santos house that gets torched doesn't increase the tally of wasted enemy gang members? That indicates the woman in Madd Doggs mansion just uses the same skin model as Denise.
It was never established why Denise was even in the East Los Santos house. She could have been kidnapped.