Druids and Shamans aren’t tree hugging hippies. They know that for some to live, others must die, whether it’s to feed themselves, or simply as a cull.
Even DEHTA, the most tree-hugging of all the Druid groups will happily murder if they believe it serves the greater good.
As an adventurer, you’re not nearly so moral. You’ve got bills to pay and sometimes those bills are paid by collecting 10 Boar Snouts.
WoW religions don’t really have the luxury of being pacifistic. Not even the Church of Light, let alone other religious sects which other racial Priests would come from (such as Night Elf Priestesses like Tyrande, or Troll Priests who gain their power from worshipping a Loa, like Bwonsamdi).
The Church of the Holy Light is about healing and protecting primarily, but sometimes the best way to heal and protect is to smite something before it becomes a threat.
Same reason why Death Knights are still slaughtering the Scarlet Crusade for Arthas in their intro, or why you can go to Alternate Universe Draenor and murder a whole bunch of Mag’har, or why characters like Illidan can be in multiple places at once, as well as being both hostile and friendly to you in those places.
The actual in-game universe simply cannot keep pace with the story. If it did, there’d be practically nothing to do in older zones as much of the conflict would have been resolved. Dark Irons are now a part of the Alliance, just as Blackrock and Laughing Skull Orcs (from AU) are part of the Horde.
WoW was never an open ended RPG though. You never had the luxury of saying “no, I don’t want to do the main storyline. I’m gunna go do everything else instead”. It isn’t Skyrim, it’s not Divinity Original Sin and it’s not the Witcher.
The mechanics of the game will always take precendence over the story. And the mechanics of the game dictate that there must always be dungeons to do, raid to raid, and boar butts to collect.
This one actually has a pretty clear explanation. When Sylvanas established the Forsaken, she sent emissaries and letters to basically everyone. Most were either sent back, or in more cases, straight up killed since pretty much every race associated Undead with the Scourge, but the Tauren, specifically Hamuul Runetotem saw the Forsaken as not monsters, but victims of the Scourge’s curse and as such, thought that working with them on a cure for Undeath was a worthy purpose.