There will be spoilers for the Power of Three and possibly Omens of the Stars too.
We all know what happened to Ashfur. He got rejected by Squirrelflight. He tried to kill her, her 'kits', and seemed totally insane, spilling all the secrets out. But why?
Perhaps he was lonely. He always loved Squirrelflight, that is certain. And she pushed him away. That would make anyone lonely, right? To see the cat you love with another mate is sure to make you feel lonely and unloved. Perhaps trying to kill Hollyleaf, Jayfeather and Lionblaze was his attempts at getting her back, though saying that, he did go about it in totally the wrong way.
Or maybe he was sad, and hated his life, and kits that could have been his. As he watched the kits grow up, surely he would be sad, and hurt, knowing that Squirrelflight had left him for Brambleclaw (BTW, I'm for BramblexSquirrel). Maybe he felt like turning against the world because he fell into a pit of sadness and despair.
Possibly he was just completely mad. Maybe his sadness, and lonliness drove him mad, and he was not thinking straight in his mad, deluded state. He may have attempted to hurt them because of irrational thought, or a deluded thought which was bent upon revenge
Another thing: On his being in StarClan. It was said that his only fault was he 'Loved too much'. That by far was not his only fault, but did only love, and he died because of his love. There is a fine line of what qualifies as Dark Forest cat and StarClan cat, but I'll leave that to be a blog for another day.