I don't really follow. I thought Darwin says that morality evolves to be more sympathetic. That he believed sympathy was the end goal of morality/people can't turn away from sympathy.
In Chapter 5 he says,
"The aid we feel impelled to give to the helpless is mainly an incidental result of the instinct of sympathy, which was originally acquired as part of the social instincts, but subsequently rendered, in the manner previously indicated, more tender and more widely diffused. Nor could we check our sympathy, even at the urging of hard reason, without deterioration in the noblest part of our nature."