Holmes was secretly transferred to a new prison last month after the assault in October.
The Colorado judge on Wednesday said evil and mental illness were not mutually exclusive.
Some had opposed the decision to seek death penalty for James Holmes.
The panel of nine women and three men found the 27-year-old guilty on all counts last month.
The mass shooting killed 12 Americans and wounded 17 more on July 20, 2012.
Holmes's legal team made the argument that he is insane and thus cannot be held guilty for his actions.
The jurors are expected to start deliberating next week to decide whether James Holmes was mentally ill.
The judge's decision came after a juror told the judge that another panelist had talked about a prosecutor's Twitter message relating to the trial.
"I saw blood pouring from his face, and I knew he got shot in the head."
9,000 potential jurors received a summons, “one of the largest pools in American history.”