function NPCKill(tParams)
{
    // EntIndexToHScript turns an entity id into a handle
    local hKiller = EntIndexToHScript(tParams.killeridx);
    local hNPC = EntIndexToHScript(tParams.entidx)
    // ([1] player) killed NPC ([67] npc_nmrih_shamblerzombie)
    printl( hKiller +" killed NPC "+ hNPC);
}
ListenToGameEvent("npc_killed", NPCKill, "YourContext");