There are all kinds of villains out there. Some just want "material things" like money or power, others are fanatical ideologues who pursue their warped ideas of justice and virtue. Lionel is sincerely the former while Lex pretends to be the latter, especially in later seasons. I think this disparity in sincerity is very important.
Lex in later seasons when trying to rationalize his actions to others phrases his misdeeds in terms of "the greater good" and "shades of grey." But we get multiple glimpses into Lex's heart and mind and so we know what he actually thinks of himself. And it's not "valiant vanguard defending humanity from superpowered individuals and living without regret." Lex, ultimately, does see himself as a bad person. He sees the exact same moral lines that Clark does and I think his respect for Clark was due to Clark's ability to stand on the right side of these lines. But even still, Lex believes, reluctantly at first and later more emphatically, that it's too late for him. His being a Luthor has condemned him to evil and there is no escaping it.
Lex's deep belief in his own evil is seen in Bizarro:
Lex: I was dead. And there was this light. And an angel. Why would she save me... after all I've done?
My interpretation of Lexmas is not that Lex chose wealth and power. No, what he chose was the "less painful path." Living a hollow life all alone is better than opening himself up to love and then losing that love. I feel this is completely vindicated in this dialogue:
Lex: And what choice would that be?
Lillian: You think the path you've chosen isn't going to hurt? It will. More than you can ever imagine. You're not the only one that's going to suffer. A field of beautiful flowers lay barren by an emptiness that can never be filled unless I keep you here. Stay with me, Alexander. Don't let the blood wash away what's left of your soul.
(my emphasis)
What Lionel taught Lex was that emotions and relationships are a weakness that others will invariably exploit. Lex in his own very specific way took this to heart. He's not afraid of people taking his money or power like Lionel is because Lex does not value those things as absolutely as Lionel does. What Lex is terrified of is getting close to another and how this brings with it endless risks of pain and loss. He craves love and companionship in a way his father never could or did but because of how he was raised, he can only see love and companionship as double-edged weapons that will inevitably wound him deeper than anything else could. Even apart from his father raising him with such toxic beliefs, Lionel's "wisdom" was seemingly vindicated with Helen.
I guess I would summarize Lex's deepest character and drives as:
- He craves love.
- He thinks he's evil, corrupted, and sullied. (which strongly negatively impacts his ability to find or maintain love)
This is in total contrast with his father. Lionel has none of these issues. He does not hate himself for what he is or what he does, because to see himself as evil, he'd have to believe in goodness. The money and the power were Lionel's only absolutes, everything else a means to those ends. Something I found really interesting is we know Lionel taught Lex a lot of Greek and Roman myths, but we know Lex is also familiar with The Bible. (see his teaching Clark about the story of David) The kind of ethos Christianity teaches is very different from that of Ancient Roman myth. It's no coincidence that Lex is attached to a belief system centered on ideas of guilt and redemption while Lionel is fixated on one all about glory and honor.
Lex's self-loathing permeated his character from start to finish while Lionel lacks the introspective ability to have such doubts.