No. She is a kind girl like Emilia. I promise you. Otherwise she will not let Subaru be able to RbD and care about him so much. She allowed him to do nearly everything except telling others RbD which will make him become a puppet of others. She even let Subaru to kill her.
And based on what those witches said, she must be a good and kind girl.
She have split personality, Satella refers to the good personality (and the original) while the Witch of Envy is the evil one. So yes as far as what has been presented so far Satella isn't evil, but the witch of envy is.