I think that Walter wanted to Crimson Stone so that there wouldn't any chance of another vampire becoming as powerful as he was. In regards to Mathias, he probably used the Crimson Stone to gain power from Walter's soul and then probably spend the next few centuries to gain more power and eventually become the Dark Lord.
I also think that the way he is finally destroyed in 1999 is that the preist from that japanese shrine seals Castlevania in the solar eclipse. Thus, cutting off Dracula from the castles power and without that he couldn't resurrect. I also think he was reincarnated because if his soul is sent to hell as usual, then there is the possibility of him being resurrected by his minions.
Another thing. Death didn't betray Dracula in 1999 because in AoS he is a boss. So he probably got sealed away with the castle.