[ItsRobinson]

PHP Code:

new MyScore[MAX_PLAYERS];
public OnPlayerDeath(playerid, killerid, reason) {
    MyScore[killerid]++;
    return 1;
}
CMD:gameover(playerid, params[])
{
    new currentwinner, score, finalwinner[24], string[128];
    foreach(Player, i)
    {
        if(MyScore(i) > score)
        {
            score = MyScore(i);
            currentwinner = i;
        }
    }
    GetPlayerName(currentwinner, finalwinner, sizeof(finalwinner));
    format(string, sizeof(string), "%s (ID %d) has won this game with a total score of %d.", finalwinner, currentwinner, score);
    return 1;
} 


This a very blank way of doing it, you'll have to integrate it with your code by yourself.
(Not tested)