In the short story, "The Split Cherry Tree" by Jesse Stuart, there is an irony in this story. What is it?

"Split Cherry Tree" is probably the most memorable short
story by the former poet laureate of Kentucky, Jesse Stuart (1907-1984). Several ironies
can be found within the story. One takes place in the first paragraph, when Dave tells
Professor Herbert that he would rather the teacher "whip me with a switch" than stay
after school. It is the breaking of a tree branch that gets Dave in trouble. Perhaps the
bigger irony in the story takes place when father Luster comes to school--packing a gun.
Luster, rough and tough and hard as nails, is ready and willing to shoot the teacher if
necessary for keeping his son, Dave, after school and causing him to miss his chores on
the farm. Luster and Professor Herbert eventually become friends, however, and all goes
well until Luster realizes that the class is about to dissect a black
snake.

    
"Don't do it," says Pa. "I believe you. I jist don't want to see you kill the black
snake. I never kill one. They are good mousers and a lot o' help to us on the farm. I
like black snakes. I jist hate to see people kill 'em. I don't allow 'em killed on my
place."
     The students look at Pa. They seem to like him better after he
said that. Pa with a gun in his pocket but a tender heart beneath his ribs for snakes,
but not for man! Pa won't whip a mule at home. He won't whip his cattle.
    
"Man can defend hisself," says Pa, "but cattle and mules can't. We have the drop on 'em.
Ain't nothin' to a man that'll beat a good pullin' mule. He ain't got th' right kind o'
a heart!"

Luster may be
willing to whip his son for misbehavior and even kill the schoolteacher for his
impropriety, but he refuses to whip animals or kill a black
snake.