[Stinged]

That's not how switch works: https://sampwiki.blast.hk/wiki/Control_Structures#switch

pawn Код:

if(issuerid != INVALID_PLAYER_ID && bodypart == BODY_PART_HEAD)
{
    new szMessage[60];
    SetPlayerHealth(playerid, 0);
    format(szMessage, sizeof(szMessage), "[(!)]%s has killed you by a headshot!",  GetPlayerNameEx(issuerid));
    SendClientMessageEx(playerid, COLOR_LIGHTBLUE, szMessage);
    format(szMessage, sizeof(szMessage), "[(!)]You killed %s with a headshot!", GetPlayerNameEx(playerid));
    SendClientMessageEx(issuerid, COLOR_LIGHTBLUE, szMessage);
}

I also fixed your message size (too small) and your playerids.
You were mixing between them and for some reason you were use a new variable (which was equal to 0)