There is yin and yang.
Cause and effect.
Check and balance.
Black and white.
In life, everything has a counter. Everyone has something that can go against the.
Day can trump night, just as night can trump day.
It is the cycle of life.
Nothing remains the same forever.
Klaus first met them when he came from the tormented world in his 4th incarnation.
They called themselves hunters. They are the blades of the heavens that, to some extent, ignore the rules and do the bidding of the heavens without any backlashes.
He fought and killed many. Those annoying hunters can just pop up randomly, but as far as he knew, they only appear when a paragon forms a soul body.
Their targets are the soul bodies belonging to those who go against the heavens. Naturally, it was Klaus and his past incarnations.
So, for a hunter to appear now, one had to say the heavens had succeeded, for the hunters he knew were some of the most powerful beings he had fought.