She's not being tried on capital murder charges which could bring the death penalty in Texas. Even premeditation isn't sufficient to get a capital murder verdict. It requires additional facts not present here.
The closest you could come to bringing a capital murder charge would be allege that she also kidnapped the victim by not freeing him or getting help. But that's kind of a stretch, too.
I'm not defending her in the least, and this crime was in many ways far more despicable than many capital murders. It's just the way the law is written.