a)My theory is that, once he regains his deck, his memory returns, and therefore realizes that he IS the devil and would start going on the rampage, therefore decade = devil.
Probably someone (Kaito?) somehow owned him before and sealed away the belt in hibiki's world so that tsukasa won't use it and travel the worlds to regain his memory/cards.
Kaito summons the dark riders to stop him but was obviously overpowered. So decade would rule the 10 worlds including his own. His armor does look evil, and it kaito seems to know a lot about his past, could have been partners that travels the world to keep things i order but decade could have been possessed?
This theory would be quite sophisticated and better...
b)Another simpler and more predictable one is that he needs more power to defeat Narutaki so he used all the 10 cards at once and couldnt handle it, just like blade's king form.
This would be disappointing if they use the same plot again....