Looking at any phenomenon we can ascribe three sets of causal conditions: necessary, contributory, sufficient. The outcome is impossible to achieve without the necessary causes, the outcome is more likely to be achieved by adding more contributory causes or by making existing contributory causes more powerful, and the outcome occurs when all the necessary causes are met and enough contributory causes are powerful enough to reach sufficient.
So, for example, we can look at the shooting in Tucson and speculate that the causes may be described something like this:
Necessary: exposed victims, gun and ammunition availability, motive, willingness to kill, ability to shoot victims.
Contributory: lack of funding for mental health care system, poor oversight of gun dealers, limitless gun and ammunition manufacturing, incendiary and objectifying public and political speech, cultural predisposition toward violent solutions to conflict, little or no conflict management education or training for children or young adults, glorification of violent warriors, nation at war, sacralizing violent warrior conduct, violent movies, violent video games and more.
Sufficient: all the necessary and some combination and level of contributory.
Understanding causation in this way helps to separate arguments that tend to place all blame on one condition or the other. We can discuss any occurrence more rationally when we don't blame it all on one cause, such as gun and ammunition availability. We can speculate about how to mitigate the chances that such tragedies will occur again by removing or reducing one or more of the necessary causes or the contributory causes. We can come to understand some of the complexity and teach ourselves how to think together toward solutions.
This also gives us all something to do. If, for example, I can vote for a person who is running for office with promises to fund more mental health outreach and treatment so that more potentially violent people can get relief and support before they descend into the stages where they might harm themselves or others, I can volunteer to help in that person's campaign, even if they have no intentions of addressing the other necessary or contributory causes. Or, if I'm a gun owner who would never hurt another, I can publicly announce that I am voluntarily giving up my gun because I am now convinced that only gun owners can bring about the changes in norms and laws that would possibly greatly reduce one of the necessary causes (necessary to the alleged shooter in Tucson, certainly, and to others such as the Virginia Tech shooter, the Columbine high school shooters, etc.). The individual opportunities for meaningful action open widely when we assess all these factors, don't they?
We will never eliminate violence, but any steps to lessen the amount and reduce the frequency of these episodes are to the public and private good. We will never eliminate cannibalism either, but we've fixed our laws and norms to attenuate that problem radically, haven't we? We will never eliminate slavery, but we've addressed most laws and most norms to greatly reduce such outbreaks globally. We have the tools and training to get these jobs done; what we lack, often, is the calm public discourse and courageous individual decisions to implement the many aspects of potential solution. We as an intelligent society full of intelligent people can approach this collaboratively if we intelligently choose to do so.