Let's look at the context of the game. It is the early 90s. You are in South Central LA (Los Santos in the game). Gang wars are out of control. Murders are common. Assault weapons are everywhere. Drugs are part of the daily routine. The LAPD (LSPD) is corrupt and uses military techniques to squelch crime.
You are Carl Johnson a black 20 something who returns to Los Santos after his Mom is murdered. Upon returning, a corrupt officer, Tenpenny (Samuel Jackson), is trying to frame you for the murder of a cop. You meet up with your brother in the hood and you and your homies go a quest to gain respect and dominance in this violence ridden society.
What must be remembered is that "Grand Theft Auto" games are satires of violent video games themselves. There is humor, irony, style, and some rather poignant themes in the game. This isn't a game like "Max Payne" or "BloodRayne" or "Silent Hill" where the violence is gratuitous and misproportional; a sort of post-modern violence for the sake of violence. The violence in "GTA: SA" is part of the fabric of the game, not an added feature laid on top of the game to make it appealing to a certain demographic.
What also must be remembered is that YOU are in control of the game. Critics love to point out that you can murder hookers for money and shoot cops amongst a group of other terrible crimes you can commit. This is true, but only if you choose to. Those who say these things are not critiquing the game, but are in fact, critiquing the people playing the game.
I think that the "GTA" games are smart, witty, and self-depricating. Like most art, it takes more than a passing glance to understand the meaning. Those who simply look at the violence in the game are missing the point. "GTA" games are successful and fun not because of the violence but because they are great games.