Kyras KNEW that Boreale would fail Kaurava campaign, thus weakening the Blood Ravens heavily. Perhaps Boreale's followers were the most loyal of the Blood Raens - those who would certainly not follow Kyras once he will defect to Chaos. Thus, he would remove the opposition in his own chapter so his ascension would be easier. Boreale, in fact, could be corrupted by Chaos himself, and he might actually be alive, perhaps returning in Dawn of War III.