Dr. Man- I got to admit that after he enhanced his own intellect to the max, he was focused on nothing more than power and world domination. In fact, he hardly listened to his subordinates' advice.
Dr. Bias- Too much intelligence does delude you out of reality after all. Some people who read too much can become like that! Dr. Bias ended up thinking he was above humanity when he wasn't.
Radiguet- It's possible he's already insane in the beginning of the series. I could count this guy as an omnicidal maniac who destroyed everything he thought was beneath him. There were times he displayed instances of insanity like when he decided to attack Juuza one-on-one despite the fact he knew Juuza was MORE powerful than he was. Also there are times he focuses TOO MUCH on strength. Okay one can credit the fact he does think but the more powerful he gets, the more insane he gets and the more he is focused on his sadism than anything else.
Gorma XV- Gorma Emperors have the trait of being crazy considering how the "You" power consume their minds, one thing Shaddam may have not been aware of. Gorma XV was plain creepy with how his unsound mind can cause too much pain! He always had his sadistic and playful outlook treating the world like his playground while Shaddam developed all the sensible plans.
Gien- He was once a human who Dinero turned into a cyborg. However he soon lost control of himself as Timeranger progressed. Before dying, his normal memory returned.
Long- I think he does make it to the list aside from being another major SOB. Driven to boredom by his immortality, his unsound mind mixed with sadism makes him really dangerous.
Ackdos Gill- While his son Warz Gill was a wimp IMO, this guy is totally crazy with how he did his final battle. He unleashed all his remaining forces to fight until the very end without thinking much.
Any more? Let me know!