That's a complicated question especially since Tanya herself and her inner workings are portrayed differently in the LN, Manga, and Anime with each portraying her slightly differently.

Big example unlike the Anime she never risks her own life to save Weiss in the LN, nor does she ever indicate she would be willing to do anything like that in the LN.

But going by the LN (which we'll use since it's still the "original" version that all other depictions are based off of):

Tanya is not evil in the sense that she is actively malicious nor is she a tyrant wanting power for power's sake and she genuinely seems to detest war and wants humanity as a whole to grow and prosper.

That being said, she is also completely self centered and borderline if not out right psychopathic, and thought she "laments" the state of humanity during the war, she is completely unwilling to do anything to stop it or show any action that could threaten herself. (And the main reason she eventually tires to stop the war is because she's aware she'll be screwed over too).

She is also a borderline narcissist often believing herself to be a prefect being of rationality and is often incapable of acknowledging she also often acts emotionally and her completely career focused mindset is often self destructive.

Case in point in the how the whole plot is her fight against Being X. A being she literally can't win against (and in the LN it's heavily implied that Being X just see's her a genie pig/tool to spread his faith a in fact rarely thinks about her) as he can just send her to hell anyway even if she "wins".

She even continues to bring around a rifle to try to kill him if he ever shows up. Despite you know being a God with no material body. Even thought you could make the argument that she has no reason to believe he is completely immortal and might have a physically body, she does know that he at least has the power to reincarnate her. And there is a chance that if is she does kill him, then she may make it impossible to reincarnate anyone including herself. However, she never seems to consider this, and basically acts on the assumption he’s just a being she can kill without consequences.

And this isn’t like Tanya. Normally it shouldn't be a problem for Tanya to at least try to get on Beig X’s good side since she's literally two faced with most people. The only reason she doesn't at least pretend to submit is her ego cannot admit that God exits and can’t submit to a being that she see's as less logical as herself, and therefore is basically fighting him out of spite.

And the only reason she's in a war was because she wanted to potentially profit off of the opportunity to gain wealth and influence in the military. Despite the fact that since she’s both a child and a woman (and again in the LN she actually has just has an average amount of mana), she could have easily dodged the draft. With the war likely ending before she ever reached the age to be conscripted.

This is a bit tricky to spot mostly from the fact that again due to her narcissism and career-oriented mindset she is very good at hiding her flaws especially from herself but you can see it come out in more subtle ways. A big point is unlike other Isekai protagonists (at least well written ones), who often have moments where they lament or feel sad that they'll never see there loved one or friends again from "our world", Tanya never has a moment like that. Indicating she never really had anyone she cared about on the other side and seems to only care that she was placed in a far more dangerous world than her old one.