He only had one tool in his toolbox. When the only tool you have is a hammer, everything looks like a nail. The growly drill sergeant from "Full Metal Jacket" was a one trick pony.
Dealing with people is a game of chess. You move emotions - yours and theirs - around like pieces on the board. Sometimes you lose an incursion and sometimes you win one.
But you never use just the one tool. Sergeant Hartman had had success with this one tool for a long time, but when the game changes you have to change too. Even just for a minute. He died because he was not that clever, and because he only had one tool in his toolbox.