Not surprisingly, the best predictors of violence in a very young child are child abuse and neglect. Children who witness violence or are themselves victimized by violence are more likely than otherwise comparable children to commit violent acts.
Some research hints that severe neglect is as much of a risk factor as physical abuse.
There is, I believe near-universal agreement that child abuse and neglect is a major determinant of youth violence, including among the very young. From work I've done with the groups as diverse as the Centers for Disease Control and the National Disease Control and the National District Attorneys Association, few have doubted the evidence on this point. Unlike many analysts, however, I also believe that positive non-parental adult influences even if they occur relatively late in the abused and neglected child's life, can stop or reverse much of the damage. The best authority on this question is undoubtedly Professor James Q. Wilson. With respect to violence by the very young, I happen to be less impressed with non-social factors than many analysts apparently are.