If the population are allowed to buy guns under civil law. If the population that has access to it exist in an area of poverty. If the law enforcement's only way of dealing with crime is to wield guns as a deterrent. If you encourage legislation that would have your school teachers equipped with guns. If your nation glorifies war, particularly those which are unjust or where the opponent has a far weaker military than yours. If you answered yes to any of these points then eventually mass murder WILL happen. When will it happen? That we can't predict, but do you know what, if you took easy access or any kind of civilian access to guns away, then I sure as hell bet that mass murder would be almost unheard of outside of a military context.
While after tragedies like the most recent in this country you never hear people that knew the person saying "yeah, I could see this happening", it's often possible to tell that something isn't right with a person. With less demonizing of mental illness and more genuine concern towards treating it, we will learn more about the worst cases before they are taken out on others. We will be able to know this is on the horizon in some cases. It isn't that simple to the point that it'll catch all of them, but some.
In the shooting in the Aurora case, the shooter's psychiatrist did have uneasy feelings about his behavior. We need to pay more attention to what is going on with the people around us. Perhaps it is wrong to say "we can predict" but it is certainly not wrong to say "perhaps we can prevent". People have a natural aversion to getting involved in the personal lives of others who seem troubled, but it would be a good thing to rethink that aversion. It is too easy to just brush it off, and then when something does happen people come up with excuses about why they did nothing. If you believe the stories coming out about this recent mass killing, it seems the family knew this young man was troubled but thought they could handle it on their own.
If it were, in every case, we would not hear about mass murder. More studies and training for professionals needs to take place for mass murder to be able to be predicted in every case. There are individuals who likely display the characteristics of someone who is likely to commit a mass murder. There are characteristics that are harder to diagnose, especially if you are not trained.
It is not possible to predict mass murder, for if it were, society would do everything in its power to see that it would never happen. After a mass murder happens, people scrutinize the situation and look for red flags. They might say that the murderer had a mental illness, that he had anger issues, that he was a drug addict, that he was a weapon collector, that he had fears of doomsday, that his favorite hobby was playing violent video games, and so on. Even if some of these red flags were noticed ahead of time, it certainly would not predict with certainty that a person is going to become a mass murdered. Many perfectly normal people have a love of video games or a hunting/gun collecting hobby or troubles in life that come and go. Therefore, despite the ability of people to notice red flags, predicting mass murder is nearly impossible to do.