Most evil regime--The Borg. It engages in serial genocides as a matter of policy and has done for centuries, destroying entire cultures and slaughtering many billions. Those that survive are tortured and mutilated, trapped in their own minds forever while made to commit the same hideous acts upon others.
Most evil individual--Kivas Fajo, a man with no compassion or loyalty of any kind. His ambitions proved fairly petty, fortunately, but he seems to lack ANY redemptive quality at all. Shinzon felt loyalty to his Reman brothers. Khan had a vision of a higher humanity. The Changelings could show gratitude and wanted the Solids controlled out of self-protection. Even the Borg saw themselves as improving the quality of life for others. Members of Section 31 were trying to defend the Federation, as were all those terrible Admirals. Nero wanted revenge from personal loss and to help his people. Fajo showed nothing even remotely like that. He felt greed, a desire to boast, and a fear that someone would take his things so he armed himself with the most vicious personal weapon he could find. He was a sociopath who treated every single person around him as an object, demonstrating not one drop of empathy.
Most evil individual--Kivas Fajo, a man with no compassion or loyalty of any kind. His ambitions proved fairly petty, fortunately, but he seems to lack ANY redemptive quality at all. Shinzon felt loyalty to his Reman brothers. Khan had a vision of a higher humanity. The Changelings could show gratitude and wanted the Solids controlled out of self-protection. Even the Borg saw themselves as improving the quality of life for others. Members of Section 31 were trying to defend the Federation, as were all those terrible Admirals. Nero wanted revenge from personal loss and to help his people. Fajo showed nothing even remotely like that. He felt greed, a desire to boast, and a fear that someone would take his things so he armed himself with the most vicious personal weapon he could find. He was a sociopath who treated every single person around him as an object, demonstrating not one drop of empathy.