It's raining heavily as Andy lies dying in the alleyway, having just been brutally stabbed by a rival gang member. From a literary standpoint, the torrential downpour provides a suitably dramatic backdrop to the last moments on earth of a young man struck down by an act of senseless violence.
Ironically, Andy has always liked the rain. And as his life ebbs away, he appreciates the cooling effect that it has upon his body, which is hot all over in the wake of the stabbing. In symbolic terms, one could argue that the rain is washing away Andy's sins, most of which he's committed as a member of a street gang called the Royals.
The rain has further significance in that it was around the time of a previous downpour that he joined the Royals. Ironically, given that he eventually dies as a result of gang violence, Andy was very happy when the Royals accepted him into their gang.
From this time, he also remembers sitting in the house of his girlfriend, Laura, watching the rain running down the windows and seeing people out in the streets run from the rain. Rain was there when Andy first joined the Royals, and it's there again when he leaves them, the latest young victim of senseless gang violence.