Angel already had an answer to this question.
The apocalypse was approaching, and the "barrier" covering Earth and blocking the Outer Deities' invasion would soon disappear. If the seven gods on Earth were still wary of one another, with evil gods lurking in the shadows, they could forget about fighting the equally numerous Outer Deities—the "Great Old Ones." Even all the gods together would probably be no match for a single Outer Deity who stood above the sequence.
Once Earth fell, the gods would lose all their anchors of faith. Even if they managed to escape, only loss of control and death would await them.
Under this pressure, the God of Combat inevitably needed to deal with the Evernight Goddess of an adjacent pathway, firmly grasping the Evernight, Death, and God of Combat pathways so he would have a chance to advance further, become a true "Great Old One," and gain the power to oppose the Outer Deities.
At a time like this, one "Great Old One" was far more useful than three Sequence 0 true gods!
Besides, ultimately, the Goddess had made the first move... When Mr. Azik had been about to become the resurrected Death, Salinger, she had intervened. While saving him, she had taken the characteristics and Uniqueness that Death had prepared as a contingency for his resurrection, securing an adjacent pathway ahead of time. This had forced the God of Combat to launch a war and settle the contest before the Goddess mastered Death's power. Otherwise, he would become the sacrifice that allowed Earth's side to assemble a Great Old One.
The Eternal Blazing Sun and the Lord of Storms presumably had similar motives, while the Earth Mother, Omebella, was the God of Combat's mother and naturally stood on his side. Only the God of Steam