New Jersey Supermarket Employee Kills 2 - Shoots Self

According to Rene Lynch of the Los Angeles Times  a super market employee walked into a New Jersey super market with an AK-47 and a automatic pistol and killed two employees before killing himself. The article goes on to say, "The incident is likely to reignite debate over guns - especially assault rifles -- and gun violence in America. It occurs just weeks after the Aurora, Colo., movie theater shooting and the Sikh temple shooting in Wisconsin."

According to Reuters  Old Bridge Mayor Owen Henry described the suspected shooter as a former marine and a disgruntled worker who had been employed by the Pathmark for just a few weeks, the Star-Ledger newspaper said.

Questions:
How does a person who has been employed for "just a few weeks" become "disgruntled" enough to shoot co-workers?

Are these shootings isolated incidents or is there "more than meets the eye"?

Will this cause citizens to rethink their position on gun ownership?

I'd like to know what you think.