Frank DeKova parlayed a sinister scowl, piercing eyes and an all-around menacing attitude into a long career of playing cold-blooded trigger-men, rampaging Indian chiefs, brutal Mexican army officers and the like. So it would probably come as a shock to those who know his work to discover that, before he became an actor, he was--of all things--a schoolteacher. Born in New York in 1910, DeKova gave up teaching for the stage, and played in many Shakespearean productions before getting work on Broadway. One of his first starring roles was in the…