I wonder if Clark hates Lex just because he knows he's a bad man, or because they are rivals for the affections of the same woman. He hates that Luthor is beating him. Not Superman him, but Clark him. Yes, seeing Lois and Lex on their date must have been doubly painful for a man doing his taxes.
He sees Lois as the most intelligent woman he knows and yet, she doesn't see through Lex's charm. At the same time, Clark doesn't want Lois to see through his Super disguise, either. This episode really pulls Clark in two directions. I agree that he only saved Lex because of Lois. If he had let Lex die, someday in the future should Lois discover that Clark is Superman she would go back and question if Clark did all that he could that night. By saving Lex's life, Clark a) doesn't allow Lex to become a martyr (or mourned), b) saves Lois from having to watch her date die, and c) also saves his own skin from a future Lois's wrath.
You capture his frustration at this decision very well.
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