Board Thread:General Discussion/@comment-29645420-20160815201731/@comment-27701762-20160815202510

If you complete the Genocide Route, Flowey basically tells you his story.

But the short version is that Flowey isn't quite Asriel. Flowey is essentially a flower that has a will to live (i.e. determination) and the "essence" of Asriel. So Flowey basically has all of Asriel's memories, but utlimately does not have a soul, meaning he cannot feel love or compassion (and presumably a lot of other things). Flowey makes multiple attempts to reignite these feelings, but eventually gives up and develops his "kill or be killed" philosophy instead, as he starts to enjoy messing with others.