Although I agree with the other answer that this is a Jackson change; that doesn't mean there isn't an in-universe justification for Sauron's actions.

> When Sauron came to kill the armies of Men and Elves, Elendil comes by. Unfortunately, Elendil was killed by Sauron's **mace**

> Why Sauron didn't just kill Isildur instead of breaking the **sword**?

Swords are considerably faster to wield than maces. A mace needs to build up kinetic energy before it can really hurt you, but a sword can stab and cut with considerably less energy put into it.

When swinging his mace, Sauron was open to being stabbed with the faster sword. So he therefore first tries to render Isildur incapable of fighting back.