Don't tell me he's mentally ill. No doubt that if he is captured, his lawyer will make that claim: That the defendant -- an otherwise wholesome, America-loving, God-Fearing, Apple-pie-eating man who helps old ladies cross the street, serves dinners at the homeless shelter, and sings Tenor in his church choir -- was overcome by temporary insanity and lacked the capacity to understand that what he was doing was wrong.
The death penalty is not a deterrent. Never was intended to be. It is the ultimate penalty for the most heinous of crimes. Those crimes that demonstrate a callous and wanton disregard for the life of another. Those crimes that, by their very nature, prove that the individual committing them can no longer be trusted to function as a normal human.
No normal human could ever conceive of the righteousness of sexually assaulting, raping, and beating a 7 year old child. NORMAL humans would give their own life to protect a 7 year old child from such harm, if need be. NORMAL humans love children, guard children, and contribute to their well-being as the child grows (while, of course, secretly wishing they'd stay a child just a little bit longer.)
Whomever did this is not a normal human. And he (or she) deserves to die.