She was told by a UT law professor during the committee hearing on HB 8 that there were severe constitutional problems with the bill, but she apparently chose to ignore that testimony.
For a discussion on the constitutional questions, here is a link to an article that lays out the constitutional problems in the issue. Below is a paragraph from the conclusion of the article:
Even without looking to international opinion, capital child-rape statutes are clearly unconstitutional. First, there is a strong national consensus against imposing the death penalty for child rape. In addition, the death penalty is a disproportionate punishment for the crime of rape, regardless of the age of the victim, because it does not cause death. Moreover, imposing the death penalty for child rape would fail to serve, and would likely inhibit, the retribution and deterrence functions of the U.S. penal system.