Both Khans were emotionally compromised by events in their lives. Neither was as sharp as he used to be. They both had tremendous flaws which showed. Both were something other than brilliant. Frankly, each in their own way, TWOK Khan and STID Khan were psychotic.
TWOK Khan spent 15 years on a planet that turned into a wasteland where he was forced to watch helplessly as his wife and 20 of his loyal followers die slow and horrible deaths. NuKhan on the other hand simply assumed that Marcus had killed every one of his followers and based his entire plan of vengeance on that assumption alone. Oh, and his assumption that his crew were killed was incorrect... TWICE! Your comparison is giving NuKhan way too much credit.