I remember watching the video where she’s speaking to the dying elf and the elf states “You can’t kill hope” and she says “Can’t I?” and turns the dying elves head toward the tree - then gives the order. I don’t believe she had any intention to burn the tree until that exchange of words.
This act was obviously pure evil but why did she choose this path?
It was all part of the Jailer’s master plan.
Because she was a spiteful jerk.
Basically, yes. When Arthas ripped her soul out with Frostmourne and forced her to be a banshee, you can compare that to a child who is abused constantly who then took their pain out on the world. It’s not an excuse, but it is an explanation.
She had intended to kill Malfurion to break the NE’s spirit, but he got away.
Burning the tree was a replacement plan.
All of the above is what she told Saurfang, but the truth was that she just wanted to kill a whole lot of people to send their spirit juice to the Jailer.
I think the lore reason is the Jailer made her do it.
My question is why not do it sooner? We’d been at war well over a decade at that point, Orgimmar and the Undercity had both been raided by the Alliance. What STOPPED us from directly attacking each others’ cities (besides plot armor, of course).
Nah that part was Elune’s master plan.
Something something blah blah shock value edgy blah.
It was a shockingly bad storyline from start to the end of the jailer.
Elune didnt cause it. She just released all those souls to the shadowlands thinking they would go to ardenweald and help her sister. Normally she keeps night elf souls around as wisps.
Sylvanas was going down the genocide path as early as Cataclysm in Gilneas. I don’t really think she needed pushing from anyone. The only thing that shook her out of it was that she didn’t want to be told what to do.
But she specifically wanted to send souls to Ardenweald. Someone had to kill those night elves or she would have no one to send! She needed the tree to burn to complete her plan.
Which is still weird, to me, since she was being told to do what she claimed to want to do anyway. It’s like toddler-logic.
“I want ice cream!”
“Okay, eat this ice cream.”
“DON’T TELL ME WHAT TO DO!”
The reason was because a muppet decided it was cool to write her as that.
Seriously it should have become a muppet special.
Also a good video I’d like to repost once again:
…no, just blind writing.
So does this mean the horde is evil?
It wasn’t a plan. It was her making the best of a terrible thing that happened. She literally said, “In the wake of tragedy, I sent forth a cascade of soul…”
Not, “I caused a tragedy to free up some souls for you.”
My lawyers are advising me to tell you “no, the Horde is not evil”.
Also, my lawyers are demonic entities from the Twisting Nether, fed with the souls of the damned I have ripped from my foes to fuel my unholy powers. So nothing odd about that.
Supposedly the cannon in Azshara. 
…although my crazy question is why do these characters of the horde follow Sylvanas? 
Elune had the power to douse the flames or do any number of other things to save the people who worship her, but she instead chose to put them to sleep so they both could not escape and would not feel the pain of being burned alive.
Having the power to save someone and actively choosing to allow them to die isn’t quite murder, but it’s close.
How did that STOP us from attacking? It was literally aimed at Stormwind. You could look through the scope! Granted, it was a goblin-built cannon, so the place it was aiming was probably the LAST place it would hit, but we’d never know because we never fired the damn thing!
And if it was to protect the Horde, where was it during the Siege of Orgimmar?!?
Bad, bad writing. Too many characters, not enough development. Just a kid bashing action figures together until something breaks.