Cold-heartedness is the noun: the quality of showing no pity or sympathy. Her cold-heartedness shocked them. It names the trait rather than the person or the act. It sounds a little formal, even literary. In ordinary speech people more often say “she is so cold-hearted” than “her cold-heartedness”. Use this when you want to talk about the hardness itself, as a thing you can point at.

冷酷；冷酷無情