Carter made a fool of himself, again. Most people know that is no way for an ex-president to behave. To do it at a funeral is beyond stupid and rude.
On his personal level I think that being Carter's administration can be summed up as a complete disaster and he knows he can not rehabilitate history, his only alternative is to attempt to slander everyone else so he doesn't look that bad compared to others.
In general I think Carter has probably always been a nut and not very Christian and is just being himself the last 15 years or so.