Garithos was the one who drove Kael'thas from the Alliance. But Quel'Thalas remained neutral for several years - even rejecting Sylvanas when she reached out to them, after which the tauren sympathized with her people's condition and welcomed them into the Horde. But remember the real reason why the blood elves joined the Horde? That was only because the night elves' Sentinel spies had established a military base, and were spying on their activities while secretly sabotaging their arcane sanctums.
I want to analyze this controversy from BOTH sides, from the blood elves and the night elves' conflicting perspectives:
The blood elves (from their POV):
- We are still neutral. We have not attacked Darnassus or Kalimdor. We did not spy on your lands - or harm the night elf people. And yet you are here uninvited, intruding upon our sovereignty without any provocation.
- How dare the night elves judge us? After they banished our people from Kalimdor thousands of years ago - denying us our birthright and heritage? Tyrande and Malfurion no longer have absolutely ANY authority to interfere in our internal affairs.
- The night elves did nothing to assist us during the Third War, or afterwards. They briefly helped Kael'thas later on only out of mutual convenience. They did nothing to help us fight back the Scourge or prevent the genocide of our people. Why should we respect their concerns now?
- The night elves took advantage of our trust in the dwarves, our former allies, by sending Anvilward, as an "ambassador", adding insult to injury. They were self-righteous cowards to spurn our ancestors, and to fear even facing or communicating with us directly. They are truly without dignity and honor. Never again will we tolerate being betrayed or manipulated, not by anyone or anything.
The night elves (from their POV):
- Kael'thas allied with the naga, our ancient and hated enemies - and killed some Alliance members while escaping, despite Tyrande and Malfurion assisting him earlier in the Eastern Kingdoms.
- We have helped the quel'dorei in the past during the Long Vigil, despite our differences. For example, Emmarel went to assist Talanas Windrunner. And Shandris Feathermoon fought beside Areiel several thousand years ago.
- The blood elves are Dath'Remar's people, and we still feel personal responsibility for their wrongful actions, as their ancestors were of our people. True, he assisted Lady Tyrande. But he and his Highborne still helped her summon the portal for the Legion, destroying countless lives and causing the Sundering. Who knows what second calamity could result from reckless use of magic, so soon after the Third War? We cannot forget how Illidan, another ally of the naga, nearly destroyed parts of the world when he attacked the Lich King from Dalaran Crater. Our duty now is to preserve the balance in all of Azeroth after the loss of our ancient immortality, and Nordrassil's near-destruction.
- The blood elves are consumed by bitter anger and hatred -- they are irrational, unstable, unpredictable and have callously exiled even the high elves, their own kin, from their lands, simply because they refuse to drain energies from mana wryms. We see them as emotionally damaged arcane addicts, who practice even darker and more dangerous magics - unlike we who resolutely shun both schools. We cannot risk them hurting others or themselves. We must keep a close eye upon them.
- Everyone spies on everyone else. The blood elves killing a couple of spies without negotiation or dialogue, spies who were not at first openly aggressive, is akin to Stormwind humans killing Cenarion Circle agents who quietly spy in their city, understandable, perhaps, but a very harsh and excessive move. Our people and the Alliance were not officially enemies to the blood elves, as Silvermoon was not then part of the Horde, and so they essentially attacked a neutral party, even if we were spying upon them.
- Of course, the Azuremyst Isle storyline. The night elves were the reason the draenei joined the Alliance. No doubt the draenei also informed them about the blood elves sabotaging the Exodar and thereby killing even more of their already near-extinct people, further spreading doubt and suspicion against their kin. But the exact timeline of events is very vague and unclear. However, Chronicle lists draenei joining the Alliance via the night elves, in the section immediately before blood elves joining Horde via Sylvanas.
Notes:
- Tyrande might not have sent the Sentinel spies. Fandral might have sent the expedition - as an Archdruid of the Cenarion Circle, he might have viewed the blood elves and their activities with deep mistrust and particular disgust, and using his long-standing and considerable influence in night elf society, quietly arranged the Sentinel spies, without informing her or the other Alliance leaders.
- The blood elves seemed neutral towards the night elves prior to this incident. Kael'thas was not hostile to Tyrande and Malfurion when he encountered them, nor they towards him. In Blood of the Highborne, the night elves are mentioned without hostility several times, one blood elf even declaring they were right regarding arcane magic use after the Sunwell was tainted. And towards the end, Liadrin hesitated and quietly mentioned that their kaldorei cousins would be surprised, after Lor'themar informed her he was negotiating membership with the Horde, to which he replied that times change and people change.