And, like any good thoery, we get it shot down in moments. Allow me to attempt to salvage the thread
What if Gavilar was going something similar to Taravingian in badness, with no hope of change? The Parshendi can't exactly walk in and tell him to stop, now can they? Unfortunately, they only have pieces of honor. The splinters aren't enough to completely destroy their personality, only enough to tilt them towards honorable intentions. They used Szeth to kill Gavilar, but for some reason totally unexplained and similar to Taravingian's purposes, they ask him to make it big and messy. Why? To get them to go to war on the Shattered Plains. Probably in hopes of getting all the armies in one place, up and ready for the final Desolation. Once it came, self-preservation would bring the ten armies together, possibly even driving them to ally with the parshendi in the event that they're not the voidbringers.
How's that sound?