Anti was being… Punished.
As much as he wished he was the boss of everything he did, he wasn’t. Anti was definitely not the one in power in Hell, there were many who were above him and that often for him into bad situations. Recently, Anti’s Mentor faded. He felt the pain of loss for the first time in his life and grew angry at his new one – with reason, since he was the one who caused his Mentor to fade in the first place. However, Anti killed his new Mentor, which did /not/ please the higher-ups in Hell.
As punishment, Anti was forced to remain in his mind space for an undetermined period of time, without any kills. He was forced into his true form, a black spiked collar on his neck, which prevented him from turning back into any other form. He was also muzzled, keeping him quiet and at bay. He had marks all over his body from the torture they made him suffer before dropping him of there.
Presently, he lay in a corner of his mind space, resting on his own. It’s not like he could do anything in the state he was in. If he even so much as tried to escape his mind space, he would be electrocuted.