Why doesn't Batman kill people, even clowns?

In Alan Moore's fatal joke, the relationship between them is expounded. In fact, Batman and the clown are the same kind of people, both of whom have experienced darkness, and both like to destroy each other spiritually to prove their "correctness". Only Batman chose justice and order, and the clown chose crime without a bottom line. The clown has always hoped to stimulate Batman to kill people to prove Batman's absurdity and hypocrisy, and Batman has always strongly sponsored Arkham madhouse to cure the clown. Once Batman kills the clown,

But the truth is, none of them can beat anyone. Batman went to the madhouse to meet the clown, and the clown showed his cards directly. "We just kept fighting until one of them killed the other." Finally, the clown kidnapped Gordon's father and daughter, Batman caught the clown, and the clown told Batman a joke. As a result, Batman and the clown laughed at the same time and reached out to catch the clown, leaving an open ending worthy of readers' deep thought: Batman caught the clown as before or Batman directly killed the clown. Of course, some people say that Batman enjoys fighting with clowns. Of course, this is also the saying that different people have different opinions.