At this point it is important to draw a distinction between the blood elves who followed Kael'thas and the naga through the portal into Outland, and the blood elves who stayed behind in Quel'thalas and Silvermoon City. They were still one faction at this point, but a number of differences were starting to appear. For one, although the blood elves in Quel'thalas were drawing on fel energies just like their brethren in Outland, they certainly weren't surrounded by demons like Illidan and all his minions all the time, not to mention the vast energies of the evil Twisting Nether, which surrounded all of Outland. Thus, the blood elves in Outland were saturated to overflowing in magic and power, while the blood elves in Quel'thalas were still rather hungry for it.
Therefore, Kael'thas thought it wise to send the gift of this captured naaru, named M'uru, back to Silvermoon City, so that his people there could have more energy to help quench their magical thirst. Soon, however, the blood elves of Quel'thalas found a way to start using this power of the Light rather than merely feeding on it, casting spells and blessings in the same way that human, dwarven, and draenei paladins could -- while the other races drew on the Light through the power of their faith, the blood elves learned to control it as it flowed through M'uru.
The first blood elf to take up this path of corrupted paladinhood was Lady Liadrin, who then founded the order of Blood Knights that became infamous throughout Azeroth and Outland alike. Thrall and the other leaders of the Horde disagreed with the methods the Blood Knights had employed, but could not deny their strategic value on the battlefield.
Thus empowered, the blood elves of Quel'thalas dreamed ever more fondly of joining their great leader in Outland one day, but as time went by, communication with their beloved prince began to drop off. Whatever was occupying his thoughts seemed to keep him too busy to communicate very much with his people back home. Some of the citizens of Silvermoon began to speak out against the practices of the new blood elf order, and even though these voices were quenched at first, the blood elves began to integrate more and more into the Horde society, and soon many of the questions regarding the rightness of what they were doing could no longer be ignored.
When the Blood Knights and other adventurers of Quel'thalas were powerful enough to come to Outland through their own power, they discovered that their leader had destroyed the natural environment around his new capital in Tempest Keep, leeching so much magical energy through his giant Mana Forges that the very land around them drained empty of life and began to fall apart. That their prince could be consumed with such lust for power at the cost of so much wanton destruction was unconscionable.
That was enough for many of the Azerothian blood elves to renounce Kael'thas and declare him their enemy, including the group known as the Scryers, now stationed in Shattrath City and sworn to assist the naaru A'dal. But many of the leaders and people of influence back in Silvermoon City remained not entirely convinced. As adventurers in the Horde, including many of their own people who had joined the Scryers, ventured into Kael'thas' fortress and uncovered his plans, it became more and more clear that their prince had undergone terrible changes and descended into madness. They fought him, but he escaped, taunting that he had sought a master with power far greater than Illidan -- Kil'jaeden, whom Illidan once served without success, and who now sought once again to destroy all life in Azeroth and Outland as the culmination of his millennia-long quest to crush the universe beneath his foot.