[Bacardi]

My example... free to try
*edit
I haven't tested

*edit
hmmm, last part of your explanation would not work in this code.
Example, it's 3rd turn -> Joe kills Bob -> Joe kills random guy -> Bob kills Joe, counter will continue normally

*edit
maybe not work at all when more players are killing each others

would not work :(

PHP Code:


StringMap StoreTurns;

public void OnPluginStart()
{
    HookEvent("player_death", player_death);
}

public OnMapStart()
{
    // clear array on map change
    if(StoreTurns != null) delete StoreTurns;

    StoreTurns = new StringMap();
}




public player_death(Event event, const char[] name, bool dontBroadcast)
{
    // follow players userid for not mix them
    int victim_userid = event.GetInt("userid");
    int attacker_userid = event.GetInt("attacker");


    int victim_index = GetClientOfUserId(victim_userid);
    int attacker_index = GetClientOfUserId(attacker_userid);

    // valid client indexs and in game
    if(0 < victim_index <= MaxClients || 0 < attacker_index <= MaxClients || !IsClientInGame(victim_index) || !IsClientInGame(attacker_index))
    {
        return;
    }

    // filter bots out, human players only
    if(IsFakeClient(victim_index) || IsFakeClient(attacker_index))
    {
        return;
    }

    
    
    

    // Find attacker in array by "userid" and collect stored data
    // data[0] == victim userid
    // data[1] == in turns counter

    int data[2];
    char key[32];
    Format(key, sizeof(key), "%i", attacker_userid);

    if(StoreTurns.GetArray(key, data, sizeof(data[])))
    {
        // if victim's userid doesn't match with stored data, reset counter
        if(data[0] != victim_userid)
        {
            data[1] = 0;
        }

        // Now delete old record and create new one
        StoreTurns.Remove(key);

        // Notice, we swap attacker and victim in data, to match next time in code cycle!!
        Format(key, sizeof(key), "%i", victim_userid);
        data[0] = attacker_userid;
        data[1]++;

        StoreTurns.SetArray(key, data, sizeof(data[]));


        // you want follow records ?
        // PrintToChatAll("%N Kills %N - counter %i", attacker_index, victim_index, data[1]);



        // These in turns was 5th time now
        if(data[1] >= 5)
        {
            // DO YOUR STUFF HERE
            PrintToChatAll("%N and %N are now enemies", victim_index, attacker_index);
            StoreTurns.Remove(key); //delete record
        }
    }
    else
    {

        // Old record not found, create new record
        // swap victim and attacker !!

        Format(key, sizeof(key), "%i", victim_userid);
        data[0] = attacker_userid;
        data[1] = 1;

        StoreTurns.SetArray(key, data, sizeof(data[]));
        // PrintToChatAll("%N Kills %N - counter %i", attacker_index, victim_index, data[1]);
    }

}