I posted this somewhere else, but I believe (or it seems to be) that your relations with a particular civ have alot of bearing on whether they 'may' surrender to you. Going to war with a civ apparently does not impact your relations to much of a degree. So if you have 'close relations' to a weak civ, then go and destroy their whole military and take a few planets, chances are they will probably surrender to you, even if you are tyranny of evil
and they are saints of the heavenly
.
My last game, playing as evil Terrans, before I went evil, the Torians declared war on me because they said it was in the economical interest to do so. And they were more powerful at the time. After 2 years of war, I was gaining ground on them and pretty much decimated their military, and they surrendered to me. I officially became evil during this time.
The Korath were powerful and evil, but I could not get on good terms with them no matter what (negatives for being weaker, militaristic, warmongers, close borders.. only plus was ethical alignment and something else). The 'war assassination event' happened and after 2 years of war there, I was gaining ground and finally they surrendered to the Drath I believe (Drath were very weak, stuck in some remote corner, but had all their trade routes going to the Korath), so they were on good terms.
So I can't say for sure, but it seems to be what I can gather about the seemingly random surrenders. In my current Suicidal Game, I will keep close tabs on all civ's relations with each other, and see if there is any type of correlation between the two aspects.
Since relations SEEM TO BE (not sure) effected solely by the +'s and -'s that you can see on the relations screen, declaring war out of the blue and decimating some civ.. then making peace for everything they have, will not make them hate you... It may add a - for warmongering, and you may have a - for ethical alignment, but if you have enough +'s in there, they will still love you regardless.
And the Torians said to the evil Terran Alliance: "Thank You for killing hundreds of billions of our people, and thank you for making us give you everything we own so we can barely survive with the few planets we have left. We feel so friendly and close to you! Let's make an alliance!"