The line seems to be drawn at choosing whether to charge the murder at a state or federal level. In Luigi's case, he is being tried federally.

Which asks the question as to why Luigi's case is being tried federally while mass casualty/terrorism crimes are only being charged at the state level.

I think texas has a death penalty for capital murder, which begins at two or more murders done at the same time. That means the el paso shooter could have gotten it. Xorollo pointed out the florida possibility.