Yes, lead poisoning could really be a cause of violent crime

[HiPointDem]

70. ...your first link...

The studies surveyed in this review indicate that clinically significant frontal lobe dysfunction is associated with aggressive dyscontrol... No study, however, shows that disorders of prefrontal cortex predict violent crime.

Methodological problems in this literature include a lack of prospective data, small subject numbers and lack of adequate controls for known violence risk factors. Study samples often draw from groups (prisoners, attorney referrals, or those with severe neurological or psychiatric illness) that do not mirror the general population or even the larger criminal population. Reports describing persons charged with violent crimes tend to cite gross measures of brain function with low specificity and questionable clinical significance, while failing sufficiently to relate the clinical data to the specific aggressive behaviours in question.

Although the bulk of research on violent and criminal behaviour points to multiple, probably interacting, causal factors, few studies attributing violent crime to frontal lobe dysfunction adequately address concurrent psychosocial variables such as emotional stress, drug and alcohol misuse, physical and sexual abuse, family breakdown, and poverty.

Case descriptions suggest that focal orbitofrontal injury specifically impairs capacities for social judgment, risk avoidance, and empathy that inhibit inappropriate or reflexive aggression. The actual frequency of violent behaviour, however, seems relatively low.

http://jnnp.bmj.com/content/71/6/720.full

that's a review of the relevant literature up to 2001.