[quote][b]Posted by:[/b] LiveAdam Near the end of Halo 2, Johnson seems to know that the Arbiter has just learnt the truth about the Halo rings and therefore asks the Arbiter to assist him in stopping Tartarus to stop the rings. Even if Johnson knew the Brutes turned on the Elites, that wouldn't be enough to assume that the Elites now want to stop the rings from being activated. I am pretty sure that even when the covenant did split, the Elites were still hostile to humans at first. I am sure there is an explanation for this, it just seemed weird when replaying Halo 2 that Johnson somehow knew the Arbiter would help him.[/quote] Johnson had a Scarab pointed right at him. If he wanted to, he could vaporize the Arbiter, and Arby knew this. It's probable that he also saw Brutes and Elites killing each other earlier. They would both benefit from helping each other, so Arbiter probably thought "Well, killing Tartarus and stopping Halo is better than getting vaporized by a Scarab, or the Halo Rings."

probabaly because he saw the arbiter not do anything to the marines right next to him.

The Covenant were not on a conquest to destroy the Universe, they believed the Rings were holy and religious, but when that was proven wrong, the Arbiter knew he had to help stop the Covenant from destroying the Universe.