A lot of times, I think the default assumption is pure power-lust, but that feels hollow to me. Take Cersei Lannister from 'A Song of Ice and Fire'. Her ruthlessness isn't born from a desire to rule for its own sake, it's a desperate, clawing need to protect her children and secure their legacy in a world that despises her and views them as illegitimate abominations. Every terrible choice is twisted maternal instinct. It's vengeance against a society that stripped her of agency and then punished her for seizing it back by any means necessary.
I've read a few regressor-style villainess manhwa where the 'evil empress' remembers a future where her kindness got her and everyone she loved killed. Her 'ruthlessness' in the new timeline is just hyper-vigilant, traumatized self-preservation. She's not scheming for fun; she's building a fortress of influence because she's the only one who knows the wolves are already at the door. That's a motivation I can viscerally understand, even if I don't condone her methods. It makes her terrifyingly human, not a cartoon villain.
Sometimes the system itself is the villain. If the only paths to survival in a cutthroat royal court are 'be crushed' or 'crush first', her decisions are a brutal arithmetic. The real tragedy kicks in when that survival-mode calculus becomes her entire personality, and she forgets what she was even trying to save in the first place.