I thought about this a great deal when I first read the manga.
If this was my senior high school class...
I find my girlfriend(at the time) first off. I shoot her in the head.
I'm playing to win, as such I need to be okay to kill. If I can kill my girlfriend, I can kill anyone. In between that and meeting up with my friends, I go out of my to destroy any redeemable part of myself. If I happen to come across a female class mate or two, I take advantage. I'll torture them. Rape them. Whatever. The more of a monster I become the more I increase my chances of survival.
Next step would be to find my small circle of friends (the 4 of us) and form an alliance, make a plan to escape, ectera. We're all really close and I'm fairly certain I wouldn't have to worry about them turning on me and that I think they'd be gullible enough to believe I wouldn't turn on them either. Of course, I would though. As soon as we started nearing the end, I'd start picking them off one by one. Making it look like an accident and what not.
Ideally this game ends with it coming down to me and my best friend. I sneak up on him when he's unarmed, with my weapon aimed at his head. I come clean, like a fucking bond villian, revealing I was the one that murdered our friends. That I planned all this for it to come down between the two of us. I say to him that the only way I'm satisfied losing is if he's the winner. I say I'm throwing the gun over to the left, we're going to scramble for it. Either I will kill him or he will kill me. I ignore his protests. I chuck the gun.
When he runs for it, thats when I pull out the other gun and shoot him in the back. He falls down, probably calls me an asshole. I apologize for lying, but to my credit I gave him a better chance then anyone else I killed.
I go on to live every single day, isolated and alone, in constant self loathing, suffering from post trumatic. But I force myself to live every single day until I draw my last breath at the age 84.
S'fucking dark right? But fuck it, I want to live. Being a good person in Battle Royale only means death.
Mind you, Shuuya and Noriko would of been dead without Shogo. Shogo wasn't a good human being either. Fucker killed his own girlfriend, knowingly got himself re-entered into the game. He could of warned everybody, saved everyones life.