^
Truth began his plans to get rid of the Brutes at the first battle of Harvest ( first contact with the Covies )
Why? for many reasons, I will now share one of them.
All Covenant races excluding Brutes were starting to lose faith in the great journey by first contact, and as the war with the Humans continued, the Covanant lost even more faith.
But the Brutes though you would not know it by looking at them ( see how they handle 343 guilty spark ) They believed in the great journey, more than the Elites did by far.
The Brutes had faith in the great journey unlike the Elites who like most Covenant races, their belief in the great journey was starting to strain.
Even before the schism, Brutes hated Elites, Elites hated Brutes, Ships under command of Elites almost never had Brutes in the ship as most of the time it caused an argument about who was in command. Same for the Brute ships. And same for Brute and Elite fleets.
But the Elites and the Brutes would never kill one another, they hated each other but they would not kill one another. Elites still believed in the Prophets and the great journey, just less than the Brutes.
So that scene when Arbiter is told the Brutes killed the Elites is his only actual character development scene in Halo 2/Halo 3. But it really isn't even character dev, he does a complete 180 in about five seconds.
So yes, that was the extent of his "development" in the Halo trilogy.