When you exit the Hotel after The Head Henchman quest, Angel will catch you reappearing and try to blackmail you into killing the Henchmen in the Park. If you say yes, she gives you her gun. If you say no, she promises she'll punish you later.
Should you accept, you need to make sure you have a Meditation Pill in your inventory. Equip the gun and the ammo Angel gave you and go to the Park. Click on any of the Henchmen there to begin.
Kill all 8 Henchmen. Your Morality will decrease slightly with each one you kill.
Afterwards, go speak to Angel in the parking lot in front of the Bank.
She takes her gun back and tells you she's impressed. As a reward, she flashes you her tits briefly. You try asking for a little more, but she tells you not to push it and that you'll meet again soon.
(Note: Before going to area 69 by taking black escort car, you will have a chance to choose whether or not to reject her quest. You can only reject this quest before starting killing Henchmen in the Park and if you reject this quest Your morality will increased 5 point. But if you have already killed Henchmen in the Park and haven’t talked to her in the parking lot in front of the Bank. You can also have your reward here.) The game maker don't want us keep the powerful weapon. :P