I think we will find some explanation of Dumbledore's behaviour in Book 6. Like it was never explained how did he hurt his hand. And that also in such a way that noone couls cure it.
Also what was it he wanted Snape to do that Snape was reluctant to do and they were fighting which Hagrid overheard.
Now this is purely a fligth of my imagination. When Dumbledore destroys the ring or when he tried to steal it something happened and Dumbledore himself became a Horocrux or something like that. Bottom line being that he knew to kill Voldemort he has to die first. and he also knew noone would kill him even to get rid of Voldemort forever. So he ordered Snape to kill him.
Another thing that always puzzled me is why Voldemort never took on Dumbledore and also why Dumbledore never tried to kill Voldemort when he was coming to power.
What do u all think