I am sure that there would've been plenty of "bad" souled vampires if being a souled vamp was a common occurance. I think that Darla would have been a bad souled vampire. She wanted to get sired again even though she had the memories of having tortured and killed thousands upon thousands of humans and happily watched Angelus rape innocent women. Her one good deed in life (staking herself so Connor would live) happened because CONNOR'S SOUL was inside of her during the pregnancy, it was his goodness affecting her (though she did try to abort him all throughout the pregnancy).
Spike was a good souled vampire because William was a good man and because soulless Spike still had a spark of goodness in him and a capacity for love.
Angel was a relatively good souled vampire because of the guilt of over a century of raping, torturing, and murdering. It changed him.
Wow, spot on! One of the best posts on here in a long time!
Also, keep in mind, Angelus was always a special case. Angelus was the
only vampire in the Buffyverse that was confirmed to be void of a trace of humanity (the Judge).
And Spike was the only vampire to
attempt to care for his family even after he lost his soul (aside from one episode of
Tales of the Vampires).
Angelus was the most vicious vampire in history (according to the Master), so Liam's soul had a lot to regret. And Spike was never especially evil, he mentioned in 2004 that he just didn't understand what it meant to be evil until he was reensouled.
I think Lawson would have been a similar case to those two, but most others could go either way.
And as for why they don't, there's only one person in the world with the capability to reensoul a vampire. Imagine if there was one cardiovascular surgeon in the world, who also happened to be a soldier, and every single patient would attempt to murder her while she was trying to heal them.