Klaus knew his parents would seek revenge when they learned of what happened to his Grandma. So he made sure they knew now so he could de-escalate things.
The last thing he wanted was to hear that his parents had died trying to get revenge on the Death Angel and Death Demon races.
The Death Angel races colluded with the Divine Angel Race and the River Angel Race to attack Goddess Nivara when she was attempting to become a Supreme.
What they did was frowned upon by many; however, it was also the best moment to kill someone. Just like how Klaus becomes super vulnerable when facing the heavens, if someone wanted to kill him, that would be the best time to do it
And so these three forces combined tried and succeeded, but only partially.
She survived but at a dreadful cost...
Her suffering afterwards came from the Death Demon race when Almos, the leader of that race, captured her when she was vulnerable and imprisoned her.
