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Darla died from syphilis after having been a sex worker for American settlers. She was probably cursing the world from her death bed. I don't think everyone would react to getting the soul the way Angel does. Many if not most would probably feel more sorry for themselves than their victims.
For what it's worth, it's implied that Darla became very hard and cynical before being changed into a vampire, which is very different from Liam (even Liam's debaucheries were based on a mix of entitlement as well as guilt for not living up to expectations), or the sensitive and sheltered soul William who'd later become Spike, and even of Drusilla. That is to say, I do see Darla With A Soul as a much harder and more ruthless person than Liam and pre-Buffy Angel as well as of William and Drusilla.
Also for what it's worth, many people enjoying the show Buffy and Angel cannot understand the harsh and often bitter views of people born before our time and how cruel (even horrific) survival was, especially for those like Darla back in her day. (It's one of the great ironies that many people going on about privilege in society are obviously some of the most privileged people to have lived on this planet even if they're not a straight white cis male, but they seem unable to understand this. Though I personally believe people will find a reason to say their life sucks with their first world problems no matter how good they have it, even if they grew up in the utopian society of Star Trek or anything the machines could dream up in The Matrix.) When I view the character of Darla, I see her in that light, with what I know of life back then. Though that said, I don't think it occurs much to the writers, so it's just how I see Darla personally.
I think she mentions that she forgot who she was as human. She didn't care for that part of her life at all, and did not keep it in mind when the soul was returned. She was much older than Angel and had no tether to the world like Angel did (once he saw Buffy), no reason to be good or different to what she was practiced in.
Liam may have had a structured moral compass in life as he had family and expectations upon him. I assume he was Catholic being Irish, is this ever suggested? His soul may have been programmed for guilt long before it was lost and returned. She was a sex worker dying from her trade and ready for the world to pay for what it did to her.
My theory is that the girl wasn't reincarnated, the Master's Dear One was, the demon, plucked from hell, and forced into the form of a mortal woman. Angel, on the other hand, was the soul of Liam forced back into Angelus.