"Could it be that computer games are so violent that they diminish killing in the mind of the child because in games characters that die come back to life?"
There is a correlation between playing violent video/computer games and heightened expressions of violence, but it's not a strong one. I imagine kids would need to be predisposed to violence through nature/nurture or a combination of both to act out such an extreme form of violence (shooting up a school). Lots of potential reasons behind this sort of violence:
Sociopathy/psychopathy. The terms are interchangeable, but have been muddled in pop culture. Psychologists disagree how many people are psychopaths, but could be around 4% of the population. Psychopathy is innate and untreatable. It doesn't always lead to violence and has different expressions, but typically a person does not have a "conscience," in other words does not feel guilt/remorse/empathy. Psychologists use the Hare psychopathy checklist-revised, now the Psychopathy Checklist—revised (PCL-R), which is a 20-item checklist, to identify whether someone meets the criteria of a psychopath. I believe psychopathy is not a personality disorder as per the DSM-5, but Antisocial Personality Disorder is, and that would be the diagnosis.
Abuse/neglect. Not all children who suffer abuse or neglect continue this cycle (my mom is an example of someone who broke the cycle), but there is a higher percentage among these groups than among children from happy/healthy/safe upbringings. The abuse is usually perpetrated within the family. Behavior in children is learned (ask any behaviorist). Violence begets violence. And yes, corporal punishment is violence and is correlated to a higher risk of behavior problems. Authoritarian parenting practices are also correlated to behavior problems.
Bullying can lead to anxiety, depression, failing grades, a sense of lost control, anger, frustration, behavior problems...
Certain types of drugs/alcohol abuse.
Those with mental health issues are more likely to be the victims of violence than the perpetrators of it, but maybe misuse or abuse of psychoactive medications?
I'm sure I could come up with further explanations if I thought about it longer. Trying to get guns out of the U.S. is pointless -- it won't happen any time soon, but there should be stricter gun laws in many states, and those who own guns legally should be extremely careful not to let them ever get into the wrong hands (children getting their hands on them, theft, burglary, drunk friends, etc.). I support Andrea's statement that guns are VERY easy to get in many, if not all states. Living in Nevada where gun laws are especially lax, it's terrifying to me how easy it is for anyone to get a gun in their possession.