motivations

In reality, however, the Glasians do have a purpose, though the Imperium (and the Glasians) remain unaware.  Tzeentch sees them as an experiment.  By driving them to incurable madness and then leaving them be, they are a sort of test bed for his sorcery and Warp-craft.  To interfere would be to tamper with the results of the testing, and that is not something Tzeentch is yet willing to do.  Why, precisely, Tzeentch would bother with this is unknown to all but him.  One definite result has been the routine massacre of millions of alien souls, and not far fewer human ones, which is always a plus to him.  However, the assaults by the Glasians have also hardened the defenses of the Imperium in the area dramatically, which does not serve his ends at all.  Such is Tzeentch.
The Lord of Change jealously guards his experiment.  He has closed all of the efforts he had begun in the sector prior to the Glasian Migrations’ arrival, and endeavored to scuttle any attempts by his rivals to start up their own efforts in the sector.  Nurgle has been most persistent; the Imperium became aware of this after stumbling across the remains of four Tzeentch-worshippers ships destroyed by the Terminus Est.  The wrecks had slammed into a microplanetoid at the border of the Naxos sector and the Hapster subsector of Cloudburst, and been found by a Naval patrol from Fabique.  Likewise, though Tzeentch regards the rise of the Free Corsair Coalition with interest, he does not yet wish to stop them.  After all, they could be a part of the outcome of his experiment, and therefore as worthy of examination as the Imperium is.  Tzeentch is less pleased that the Orks of Gorkypark and other, smaller Orkholds remain in his way.  There is nothing to learn from Orks, and they are but a potential contamination in his experimental data.
Tzeentch, however, is aware that his project may not be infinitely sustainable.  The Glasians come from other galaxies, and though they are fleeing the Tyranids, that suggests that their numbers will eventually run out.  The Tyranids do not leave survivors.  Though the time-stasis Warp barrier he uses to contain the Glasian fleets is holding, and Abbadon’s little Crusade may allow him to strengthen it, the intermittent trickle of trans-galactic Cylinders will eventually end.  When that happens, he shall carefully use up his supply of trapped alien ships, then plunge the sector into the flesh change.  That is a fitting reward for the humans who have so thoughtfully served as his ideal test subjects.  Tzeentch estimates that the flow of Glasian assets from their home galaxy will run out in eighty four Terran standard years, and his stockpile, thus reinforced, will run out in eight hundred.