She has no idea who Xavier is before the battle starts so she would indeed be left completely in the dark about him and his capabilities. She will misread him thanks to that and perhaps he can be clever about covering up his stat boosts. She wants to hit hard enough to knock him out of the fight, not obliterate him, so she could figure something out about him and might enjoy the fight too.

She wouldn't come back to life right then and there, as she'd come back later on to hunt him down when they could both unleash their full potential. Unsure if this is an actual power of hers or simply because they're in the multiverse and people tend to spawn in again at one point or another. If she is able to have her dead body come back to life rather than simply spawn in, she'd likely require enough time to do so until after Xavier is already long gone and certain conditions. Perhaps his energy in her body halts her ability to come back to life, but his energy fades away from her to allow her to do so. 

That gives me an idea. If it stops her from coming back to life for a bit, perhaps his energy is sort of like her kryptonite able to break through her defenses far more easily than normal attacks. So while she thought she could block a nuclear explosion, her defensive shielding couldn't protect against his pin point attack somewhere that breaks through. She could evolve her defenses somehow to protect herself from it later on, of course, but simply couldn't do so thanks to how sudden and underhanded he was with that attack.

With her warriors pride she's probably not angry with him for killing her, being more interested in him if anything. Although if you were dead set on her being very heroic, she'd of course have to be more upset than anything else that she allowed the village to be massacred thanks to her carelessness.