public OnPlayerDeath(playerid, killerid, reason)
{
    new player[24];  // ID of player that died
    new killer[24];  // ID of killer
    GetPlayerName(playerid, player, sizeof(player)); // Player that died name
    GetPlayerName(killerid, killer, sizeof(killer));  // Killers name

    // Use a GameText or SendClientMessage to send the messages you want to killerid or playerid.
    // Or use SendClientMessage or SendClientMessageToAll, whatever you want to do is your choice with messages

    // To update score:
    SetPlayerScore(killerid, GetPlayerScore(killerid) + 1);
}