Forcing its way into Zero's mind from the very first day was intentional. In fact, its original plan was to completely possess his body and use Zero's body to destroy the Oscuro group which slaughtered its race, but unfortunately, it ended up dying before it could fulfill its main goal. Unable to accept its death which meant the death of its once-mighty race, the dusk demon transferred its consciousness into Zero's mind.
Within this time, its hatred for the Oscuro group lessened ever since it realized that the four heirs of the Oscuro group are more different from their diabolical parents. Its plan to destroy the group soon faded slowly and as it loosened up and let down its guard, Zero was able to read its thoughts while he watched the events of the past. As much as he would love to condemn the dusk demon and completely destroy it before it harms anyone, he couldn't help but accept that the dusk demon's hatred towards the Oscuro group is justifiable.