A part of Lex told him that he should not conduct social experiments on immensely strong cultivators who could influence the fate of his race over the entire universe. That part was his brain. But at the same time another part of him argued, like a tiny devil sitting on his shoulder wearing the Notorious Anonymity mask whispering in his ear, that if Cassandra were so easily influenced then even if he never did anything, she would likely go astray down the line.
He should have confidence in her ability to think for herself and make her own decisions. Moreover, since this was a matter very much related to him as it would influence his entire race, he was entirely eligible to have an opinion of it. His experiences on Earth, however limited they may be, told him that anything easily given is often taken advantage of, and not valued.