Sylvanas is not just evil for the lulz; she is, in her mind at least, trying to accomplish something that she sees as good for all the people of Azeroth. When she died at the end of Wrath, I believe her soul was sent to the Maw, where she was intended to be held for all eternity. Of course, the val’kyr pulled her out of there, but the experience has been her driving motivation since then.
Sylvanas’s goal, I believe, is to overthrow the entire hierarchy of the Shadowlands. She does not believe the Arbiter, the being that determines where someone’s soul goes after they die, should have that authority. She finds it unfair that she was sentenced to the Maw, and in her mind, she’s going to “save” us all from this being. Her goal is to destroy this system so that those who die are not under anyone’s power or authority.
So what was Sylvanas doing throughout Battle for Azeroth? It seems, despite her stated motivations in A Good War, that her real motivation was just to kill as many people as possible. This is based on her dialogue and others’ dialogue about her at the end of the War Campaign. I’m not sure, and I have no evidence for this, but I suspect that each soul who died in the war somehow empowered her and allowed her to accomplish the things she does in the Shadowlands trailer, as well as gave her the mysterious new power mentioned by Jaina, Thalyssra, and Lor’themar.
Based on her dialogue and her actions, it’s clear that Sylvanas places no value on life. I think this is because she recognizes that everything dies eventually, so she sees life as a temporary, transient state. That’s why she doesn’t feel guilty about any of the people she kills. From her perspective, everyone will be dead eventually; in her opinion, it makes no moral difference if it happens now or in a few decades. I think this is what she means when she says that all will serve death. It’s not her goal, but a statement of fact.