Liam was a lot more complex than people assume from the most vapid reading of the flashbacks. There are hints of it.
The biggest one being the line about wanting to the see the world and Angel telling Buffy how he wished to meet someone interesting (which explains why he went looking in all the wrong places), but this is at the stage where Liam is so unfulfilled by his life (there are lots of hints of that) that he's in a full downward spiral. As Liam says, he feels he can never get his father's approval, so he lives down to every low his father expects of him. He also hints in Spin the Bottle that religious hypocrisy of some sort is an issue Liam had with his father. It's mentioned that Liam's father is a middle class silk and linen merchant (has pretensions that Liam calls him out on concerning multiple servants instead of their one and eating with his hands despite owning silverware--note that provided tableware, rather than a B.Y.O. knife or a cadena pouch, was still a relatively new thing in the 18th century with the fork not even having widespread use until the 18th-19th centuries--it was still deemed unmanly by some). If Liam wanted to travel the world and pursue art, culture and languages (which appear to have been his talents and interests), he would have been very smothered if he were being forced into his father's business and staying in Galway. We do see that Liam has a heart, as he cares about his sister, Kathy. Liam was looking for a way out of Galway and we see that in his words to Darla.
Of course, we also see Liam's father as a disappointed father with an out-of-control son (how much of it is due to him being stifled and unable to get any support for what he wanted for himself?), which Angel later sees the other side of when he's father to Connor.
I.Q. is also inherited. Angelus and Angel are incredibly intelligent (very near omnilingual, artistic, a philosophical bookworm and an eidetic memory), which actually points to Liam being incredibly stifled, as a more-than-superficial reading of his flashbacks would suggest. This is hinted at in the lines criticizing his father's lack of culture and pretensions, as well as his desire to see the world instead of following in his father's footsteps as a middle-class linen and silk merchant in Galway with "simpering morons" he didn't find intelligent or interesting (and thus was looking in all the wrong places for something different and for someone to take him away). When Harmony was sired, her I.Q. didn't improve. A demon only has so much to work with. And yet, even a dingbat like the human Harmony would have been horrified about the demon committing murder, even if the two can hardly be told apart. Liam gave Angelus a lot to work with because he wasn't a normal idiot and he was angry, even if the moral center and desire to be/do good of Angel was always there deep down.
