I have 2 theories on why Lex made clone of Lana, one was for scientific reasons but crossed a line; plus, he was obsessed with Lana, so he probably wanted her as a sex-toy before deciding he wanted the real thing.
In his fall from Grace Lex preformed one of the 7 Deadly Sins
Pride: Lex become obsessed with controlling metahumans creating an army for his own personal gain.
Avarice: Lex began to abuse his wealth.
Lust: Lex became obsessed with Lana and wanted her for himself, he even made a clone of her.
Envy: Lex became envious of Clark's relationship with his adopted father Jonathan, and with Lana.
Gluttony: Lex wanted control and power, and he also wanted Lana not out of love but obsession.
Wraith: Lex could turn abusive and brutal.
Sloth: When he got his powers Clark learned responsibility, while Lex didn't learn responsibility, and instead of earning Lana's love he acted shamefully to gain it, blackmailing Simone to break up Clark and Lana, putting Lana in danger to save her and look more reliable to her, sociopathic faked her pregnancy to coerce her into marrying him, then cruelly faked her miscarriage to cover up his sin. But Lana eventually learned her pregnancy was faked and realized what else Lex did to her and would hate him seeing him for the monster he truly was.