How could they not be? They are trained to kill people. Not saying they can't control themselves any less than someone else could. But when it comes down to it they are conditioned to be violent, so as a trained real life soldier, from day one you are told kill or be killed, and I don't see how they couldn't be considered more violent in general than civilians. There are always exceptions but overall I would have to say yes and would be worried about our ability to defend our country if it were any other way.
Yes, I do believe that in many cases soldiers are more violent than the general populous, but I don't necessarily believe this is because they are soldiers. I think that people with more violent tendencies that have closed off other career options because of them often will join the military. It is a case of correlation does not equal causation. The military, if anything, works to channel and restrain those violent tendencies.
All soldiers are not psychopaths. I'm an ex-soldier myself. Still, it's been shown that soldiers are more prone to violence than average due to their traumatic experiences, and it doesn't help that we were trained to kill other people. The military is an institution of violence, so it's not surprising that its members can be prone to violence.
Soldiers are not more violent than other people. They choose to enter the military to defend their country (as well as to get paid to do a job and get benefits) but not so that they can express violent behavior. Soldiers are often traumatized by their experiences in war zones and that indicates that they are deeply affected by the killing that goes on in war. Also, there are many returning soldiers that commit suicide when they return home.