There is a clear difference between BfA Sylvanas and Legion Sylvanas but there is also a clear difference between all of her iterations honestly.

But even with Legion it made it pretty clear what Sylvanas was willing to do to achieve her goals. https://twitter.com/makaniart/status/1279647959450136579

Legion was doing it's best to market Sylvanas as a hero-character when in reality she was never really that. Sylvanas up until BfA was always in the middle of being bad & good a very dark Anti-Hero.

Classic even made it pretty clear the Horde was a means an end for her.

Wrath was about her revenge.
Cata was about her new goal.
Legion showed us the merits to that goal but also her new position as Warchief - a position she should've never had.

BfA could've done a better job but I do think she was always intended to be a villain at some point.

This flip-flopping of back and forth between Not-So-Evil and Evil can really hurt on what you're trying to portray for a villain, anti-hero, or hero.

Sylvanas was shown in Legion to care about the Horde to some degree and then BfA turns around and throws that away.

Sylvanas should be a villain and they should have fun with it. Instead of going back and forth and remaining undecided on what she is for Warcraft.

Gul'dan, Garrosh, Azshara, the Lich King all really fun villains who got to have fun being villains.