If you think Sylvanas is just evil, then you must have a very narrow mind. Everything lately points to the opposite.
Let us review Sylvanas
Legion Intro - She shows clear respect for varian, and even saves his life by attacking an infernal that was no threat to heir own airship.
Broken shore - She VISIBLE hesitates to call for a retreat, despite a direct order from her warchief, only when he pleads for her to save lives does she call the retreat, using her val'kyr to carry wounded soldiers to safety, and later on the boat when she realises the consequence of her actions, she again show visible regret.
She is reluctant as all hell to pick up the mantle of warchief, but it is vol'jins dying wish that she does so.
Broken Isles - Here she clearly had an ulterior motive from the beginning, she has learnt of helya beforehand and she seeks to strike a bargain, because she has seen what awaits beyond undead, and she fears it, as would anyone. Her actions are cruel, crude and desperate, and because of this, she does also fail.
Before the Storm Prologue Chapter - She shows no real affection for the horde, displays clear annoyance over her being warchief and she is deeply offended by the desolate council and their will to not extend their lives any further. She also seems intent on bringing the war to the Alliance, probably more a matter of wanting Greymanes head on a platter.
- Now here we have a gap in the story, because some major shit clearly goes down between here and BFA main story with zandalar and kul'tiras.
But in the BFA Cinematic, we can see a horde ravaged by internal conflict and distrust, they lack zeal, they lack motivaiton and they lack belief in their cause. And this is apparent more so in Sylvanas than anyone else, she show genuine concern mixed with anger over the situation and eventually, she snaps. She goes into full on offense and disregards her own safety completely, she singlehandedly decimates several alliance soldiers and with a howling banshee cry she echoes one of the, if not THE most genuine "FOR THE HORDE" I've ever heard in Warcraft history. And you can SEE the light return to Saurfangs eyes, you can see him echo her zeal, you can see that he now see the Warchief in her. I never thought Sylvanas was fit to be warchief, I've for the longest time thought that an old school Thrall needed to return.
But you know what? The sylvanas I saw in that cinematic? I would follow her into battle any day. She was a true leader from that moment onwards, and you could tell, she now felt the horde for what it is. Her misfit extended Family.
Sylvanas is being taken in an amazing direction, and I applaud Blizzard for finally casting aside tropes and bullshit hollywood storyline thinking and going for something far more subtle, but much more effective.
This is very clear to someone with many years of fantasy reading and character development behind them. There have been subtle hints for a long time that Sylvanas is growing into something more than a powerhungry undead afraid of her final death.