I was playing a game recently (Large map, 9 civs) in which the Korath were doing particularly well.  After a certain point, several other races surrendered to them in sequence, leaving the Korath with near total dominance.

All well and good, but very puzzling.  Leader after leader would send a diplomatic message, roughly: "We're losing, so we've decided to surrender to the Korath, we hope they'll be merciful."

But... they have the stated public aim of wiping out all other species, and the track record to prove it!  Noone in their right minds would surrender to the Korath, surely, under any circumstances?

I would suggest that any race with the Super Annihilator special ability be automatically barred from accepting the surrenders of other races.