How did Cade Skywalker defeat Darth Krayt?

Krayt was powerful, but he was not a young man. His lifespan would be limited, even with the ability to cheat death, and he could only "teach [Cade] the ways of the darkness until I become dust" if Cade was willing for that to happen. He could not forcibly turn him and Cade would not have been immune to the toxin that was meant to destroy Coruscant unless he was a Sith. Cade had previously proven himself as Krayt's #1 enemy and pretty much the only Force user who could take up Krayt's mantle. Krayt had already killed his #1 Sith and none of the other Jedi were up for the task.

Basically, Krayt had two options: Turn Cade and make him his apprentice, or spend the rest of his life fighting Cade and Cade's allies who would spend their lives fighting to the death to see that Krayt's vision would never come to pass. He chose to invest his creds in the first option in an All or Nothing hand of sabacc.

That's my take on it, anyway.

 

Krayt took a page from Abeloth's Necronomicon. Well, actually a couplet. You all know which one.

 

This!

The parallels with Wyyrlok's death were intentional, and designed to show that both suffered from the same fatal flaw.

Blame Dark Horse for that. They cancelled Legacy far, far too early.

 

Krayt was just an overconfident man, at the end of the day.

Look at how Palpatine died in ROTJ... being tossed off a cliff by an injured, handless cyborg on life support.