Don't get me wrong, I think she's terrible in the grand scheme of things. I just think that humanity being descendants of the Nephalem have more interests that align with Lilith compared to Mephisto and that might ultimately lead to a change in the never ending eternal conflict. For example, I think she would have wanted to protect Sanctuary itself, whereas both the heavens and the hells wouldn't give two shits about destroying it if it served their purposes. I also disagree about the part where she didn't care about humanity, because humanity is just essentially the children of her spawn. Plus, wasn't Uldyssian a human who later regained his Nephalem powers? So its there, just hidden by the Worldstone tweaks that Inarius did.

and we see that cruelty and malice with how she dealt with Inarius

Lilith isn't great...I just don't think Mephisto is the answer. That's what I mean by choosing the lesser evil...he's the worse choice out of the two. At least humanity has a bond with Lilith through being part of her extended family.