Why does Botticelli help Despereaux?

Botticelli, the evil, one-eyed rat, is constitutionally incapable of helping anyone but himself. This is one bad rat we're dealing with here. So when he offers to lead Despereaux to Princess Pea, we know straight away that he's up to no good. With a perfectly straight face, Botticelli tells the little mouse that he wants to help him find the princess, to show Despereaux that not all rats are bad. Not all rats are bad, of course, but Botticelli certainly is all bad. He hasn't the slightest intention of helping Despereaux; he simply wants to kill him and then gobble him up.
But as Botticelli leads Despereaux deeper into the dank, dark depths of the dungeon, he finds he's got competition. Other rats are sniffing around in the darkness, dying to get their greedy chops around the little mouse. Botticelli gets them to back off; this is his lunch, no one else's. But in any case, Botticelli is to be deprived of his sumptuous meal. For one thing, he's been put off by the pathetic sight of a fellow rat—Roscuro, no less—blubbering at the scent of Despereaux, which triggers memories of that sad and terrible night when the queen died. Botticelli's previously ravenous appetite has also been dimmed by Despereaux's kind, forgiving nature. This rat may be hungry, but he's not about to feast on something that tastes of goodness and forgiveness. The very thought is simply too disgusting to contemplate. So he slinks dejectedly back into the subterranean murk with the other rats.