Remilia is hugely manipulative, using her superior knowledge to play everyone around her like a fiddle. The only thing keeping her from being correctly judged as a monster is her utter devotion to 'Emi', which constrains her to take more socially-acceptable courses of action.
It's why this series is just so, so good. Remilia is absolutely a villain, who only cares about her own selfish desires and goes to great lengths to fulfill them. She has not really changed from the original character, who summoned the demon lord to ravage the world.
Luckily for everyone who isn't the Star Maiden and her harem, her desires are currently focused on pleasing a genuinely good person. So instead of drowning the world in fire and blood to get her revenge, she's giving everyone the happiest of endings so that she can usurp the "heroine"'s very reason for existence.