I don't see where it says that Housel has the mental capacity of a 7-year-old. It does say, on the site kat linked to, that Housel picked up Jeanne Drew at a truck stop; assuming that "picked up" means that he was driving, he at least had enough mental capacity to be able to operate a motor vehicle without attracting undue attention while the crime was in progress.

But if he's really so severely mentally ill, why was he sent to jail rather than to an institution? Why aren't they appealing to have the sentence changed to "guilty but mentally ill" so he can get some help? ...Perhaps, without the infusion of alcohol, he's not sufficiently impaired for that? Note this excerpt from the Amnesty briefing posted by Fionn:

"...two mental health experts have concluded that Tracy Housel suffers from brain damage and psychological impairment, which combined with his drug and alcohol abuse, "substantially impaired" Housel's ability to recognize the criminality of his conduct. The state has introduced no expert testimony to rebut this evidence, which could have been used to present a defence of temporary insanity. Instead, his lawyer allowed Housel to plead guilty to capital murder."

Temporary insanity. Now, I infer from the above that while he'd been sitting in jail without the aggravating circumstance of drug and alcohol abuse available to him, his ability to recognize the criminality of his conduct was less substantially impaired. And when it came time for him to enter a plea, he pleaded guilty. Whether or not he knew what he was doing at the time of the murder, he had a better idea later of what he had done that day.

That doesn't speak to the question of rightness or wrongness in executing him but I'm just saying that, based on what little info has been provided in this thread, I don't envision Housel as someone who hasn't got a clue as to why he's on death row.