Souther never hated any of the children he enslaved. It's because his belief that love would only hurt them, he was teaching them hate so that they'd become stronger people and save them from the deeper suffering that love would cause them. When he remembered love, he became as he was when he was a child. And now that he passed away, he came to Gensokyo (probably through the Palace of Earth Spirits) and started causing fun-loving incidents with Remilia as a way to enjoy his lost childhood.