Muzan Kibutsuji:
"I'm a perfect being, and your human morals don't apply to me, you low-life worm"
(Probably swats at him and tries to crush him)
Emperor Palpatine:
*Cackling* "He,he,he! You've finally figured this out, my Little, Green Friend!"
(Aaaand probably just roasts him with his force lightning)
Kenjaku:
"Oh, I wouldn't call myself 'Evil', I'm just curious and knowledge craving."
Aku:
"Well, Yesss! I am the Lord of all Pain and Evil in this World, and I'll make the whole world bow down to my bidding, now grovel before your new lord, you petulant pest! HEHEEEEHEHEEEE!!!"
Femto:
"It's just all for my dream, and dreams require sacrifices..."
(Just ignores him, because he doesn't really care about some bugs telling him he is a bad friend)