Neither the hero nor his mage buddy are assholes, they want to kill Maou only to end the chaos that happen by him existing.
Also, Tooru has no special power, but her knowledge of the series help out a lot.
For example, she know Maou can read someone's thoughts by focusing, so when he noticed he's trying to secretly do that, she fill her thoughts with praises/adoration of him. (he's like "I regretted doing that" in his head)