Even if Twomey was behind or ordered the Kingsmills massacre, there’s no evidence it was due to a desire to “exterminate all Protestants”. It was clearly to bring Protestant paramilitaries to their senses, and show them where this was headed. Savage as it was, it had the desired effect.
For a so called Irish Republican, you seem to have a very blinkered Anglophile view of Irish history. Jeremy Corbynist who I suppose is also an Irish Republican as he met with a few IRA members.