he has good skin, some meat on his bones, we don't see his teeth , but he didn't act like he was speeding.  to the contrary, he looks as if things were happening faster than he could process - his eyes are blank - a tweaker usually looks a little more angry in their mug shots = this guy is lost.   
what he did was terrible, no doubt, but mitigating circumstances were very real.  he believed he was helping her - not the sharpest knife, but it is what it is. and she bought the nail gun.. she had been soliciting people on line to kill her. this guy  didn't have that sense of a 10 yr old. at one point he stopped , and she told him to keep on putting nails in her head. she's mentally incompetent too. perfect  storm says it well.
 as to  what you "suggest" - no offense,  but it matters little to me...i can pity  and have sympathy for whoever my heart tells me too.  sounds like you've had little exposure to someone truly mentally ill. or maybe you just assumed they were a tweaker.