Why are the knight's trials easier than the other two classes? Because Kuros is already a knight and, despite his amnesia, knows how to get through them already.
Malkil having Kuros chained up in Piedup's throne room alongside him in the game over cutscene could just be a case of Never Say "Die" that was common with NES games. However, we see from the intro and the ending that killing Malkil is only a temporary setback for him, as his spirit survives long enough for him to create a new body. Since this probably falls under the category of necromancy, it's entirely possible that if Kuros dies, Malkil has his corpse recovered (not hugely difficult for him, since he is now the de facto ruler of Piedup) and resurrects Kuros just to torture him for all eternity.
