That was a taste of our power, humans. If you want more, I'll be waiting.
Decimus never thought to question the status quo until Atriox decided to break away from the Covenant religion. He respects strength over everything else, and Atriox's continued successes earned Decimus' loyalty to him. He is known to be very ruthless, as he would kill his own troops if they displeased him, as said by certain Jiralhanae in Halo Wars 2.
Decimus holds some resentment against the Sangheili due to how they treated the Jiralhanae during the Human-Covenant war. As one of Atriox's generals, he takes a cruel joy in ordering the Sangheili under his command to take part in perilous missions.