I went and verified – according to the 5th Erin Hunter chat, Mudclaw is in StarClan, because he believed he was being robbed of his code-given leadership. While I have nothing against Tallstar for acting on his vision, Mudclaw would probably have been an okay leader. If he was worried about violence, Onestar launched an unprovoked attack on ThunderClan at least once, so.
[member=55]fireheart[/member] I'm about to start book 4 of DotC and I'm not really sure what they're trying to do with Clear Sky's character. Hopefully that will clear up soon! It'd be kind of cool to have an evil Clan Founder tho!