You're going to need a bit of code inside the spawnplayer() too, to reset the killstreak counter upon respawn. (thats the lazy way, the better way would be to put it in playerkilled, spawnspectator and spawnintermission but i digress).
Furthermore, judging by the script you've showed, you're running it as a plugin, and only after the playerkilled callback has finished. This callback takes 2 seconds to finish, so your killstreak messages will be delayed. Probably not what you want.