I never liked what blizzard did there. but they stated, they wanted the blood elves to be a lot more aggressive, more horde like.
This is an aggressors, firstly, you are friendly with the alliance, while you don’t appreciate spies, you don’t go and savagely execute them on the spot, that is savage and aggressive. It fits the horde more.
It is a far cry from the high class noble elf the Thalassians were before…and this is what they wanted to achieve in a respect, to make them fit the horde a bit more, painfully aware of how very un-horde like elves were… ofc, being a core alliance race - I mean you really don’t get more alliance than high elves - they are basically humans at the most excellent level, everything humans do or can do, they do the best version of. This is the type set for them.
Taking them over was like having humans join the horde, - but this was felt necessary to make the horde more attractive and it worked. Most mmos actually have humans on both sides, because of how popular they are, Warcraft decided to solve the population crises by taking the alliance elves over.
The only reason we didn’t get humans on the horde is because they wanted the factions to have two very separate races back then.
If the same was happening now, it would be a faction of humans like the Alterac ones going horde, or the Lordaeron survivors.