Heretic Leader suggested Arby to ask Guilty Spark about Halo and it's true purpose, Guilty Spark says [i]more questions? splendid, I'll be happy to assist[/i] than she's immediately cut off by a barriage of Plasma bolts fired by the Heretic Leader.
Why would he suggest to Arby to ask Guilty Spark about Halo if he was going to shoot him right after? If he wouldn't have opened fire on him, Arby could have learned the truth about Halo (then again, Arby still wouldn't have believed it the first time)
[b]Or[/b] what if he only suggested it so he can whip out his Plasma Rifles while Arby was temporarily distracted?
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I actually just read about this on a Headscratchers page on TV Tropes. here's the copypasta for both of our convenience. At that point, the Arbiter was dead set on getting his honor back and was very much on the side of the Covenant. It's likely the Arbiter wouldn't have listened to him at that point. It wasn't until the Prophets gave the order to kill all Elites that the Arbiter realized the Covenant wasn't all it was cracked up to be. There's also the issue that the Arbiter had just killed all the heretics in the area, many of whom 'Refumee may have actually known personally; revenge is an additional motive to kill the Arbiter.