In this paper, he detailed a set of three behavioral characteristics that, if found present together in a person, he claimed were to be associated with later violent tendencies. It should be noted that Macdonald focused on hospitalized patients who had a history of making threats to kill, not patients who had actually killed. Some studies have found statistical significance to the Triad, and some studies have not.
The traits, in no particular order, are:
If a child wets the bed past the age of 5, Macdonald found this to be significant. Two psychiatrists (Hellman and Blackman), claimed that enuresis—the act of voiding urine while asleep—was a form of sadism or hostility, because the act of voiding in fantasy was equated with “damaging and destroying."
More up-to-date research has subsequently discounted associating bedwetting with violent tendencies, but does make the point that bedwetting past the age of five can be humiliating for the child, depending on how the child is treated by parental figures for doing so. If belittled or treated cruelly, the child might then be more inclined to engage in the other aspects of the triad as an outlet for their frustration.
2) Animal Cruelty
Torturing animals can be seen as a precursor or rehearsal for killing humans. Torturing any animal is bad, but messing with dogs and cats is particularly so, because they are seen as more humanlike due to being pets. Toads, turtles, worms and the like don't seem to violate that human-pet connection as much.
Some psychopaths engage in animal cruelty as a way to vent frustrations, since in childhood, they could not retaliate toward those who humiliated them. So they select vulnerable animals, seeing them as weaker. It's future victim selection at a young age. Studies have been done that prove that those killers who engage in animal cruelty often used the same method on their victims.
It doesn't have to be huge fires to be an outlet for aggression. Trash cans, small flame throwers, homemade "bombs"—they all serve their purpose, just as setting fire to a building or car does.