Oh, yeah, bruh. Alliance was horrible before cross faction. I know because I played both this expansion, sometimes simultaneously. Generally speaking the play was like night and day. Alliance pathed poorly, did not skip the right packs, missed a lot of kicks, etc. and just in general was pretty bad.
In my experience, I have always found the lower the key and the less skilled the player the more likely the player was to be toxic. The higher the key the more likely everyone is to be nice, and even just walk away from a key thanking everyone for their effort and that kind of thing. There are always exceptions though, lol. If anyone ever whispered me telling me I was a bad healer, I would always look them up–for two reasons: First to see if they had valid criticism and second to get a fix on the toxic person’s personality. Typically the toxic person’s track record would show inconsistent play, lower keys, poor log parses, etc which often gave me the feeling I may not have played perfectly, but that the toxicity was probably a result of frustration on the toxic person’s part, their poor play, and wanting to excel without much of an effort or time commitment on their part. A lot of people are quick to blame others and their habit of doing so means they legit never learn from their own mistakes and just want an easy out to put their egos to rest.
Horde are typically better players, so I disagree with that assertion.