ON MONDAY, James E. Brown, a 40-year-old security guard, was shot and killed as he attempted to stop a bank robbery at the Suburban Bank and Trust Company in Seat Pleasant. Mr. Brown, a husband and the father of young children, was the victim of a crime that is occurring with alarming frequency at a time when other violent crimes are declining. A killing is unusual in the course of a bank robbery -- it happens only about once in 200 cases -- but danger always exists, and witnesses and bank employees often suffer assaults and continuing psychological trauma.

This year, there have already been twice as many bank robberies in the District as there were by the end of February last year. Nationally, the figures follow the same pattern. Before the '60s, the FBI reported fewer than 500 bank robberies a year; by 1981, there were 8,000.

Unlike their glamorized prototypes, Butch Cassidy, Jesse James, and Bonnie and Clyde, today's bank robbers are recognized as a not very smart group. They usually fail to take into account new security measures such as videotape equipment, exploding money bags and timed devices that spray dye on the fleeing hold-up man. According to the Justice Department's Bureau of Justice Statistics, which produced a profile of the typical bank robber, 76 percent used no disguises, 86 percent never inspected the bank before the offense and 95 percent had no long-range plans to avoid apprehension and spend the money without being noticed. The typical criminal in this category is a young, male recidivist who is an addict. A majority have already served time in prison for a serious offense.

Fortunately, most are caught, with arrests being made in two out of three cases. And they are sentenced to long prison terms. We hope this fate awaits the criminals who are increasingly assaulting area banks. Especially deserving of it is the vicious gunman who killed Mr. Brown the other day.