Is there a word to describe someone's willingness to commit horrific acts?

Example: "It used to be hard for him to take the life of an innocent, but over time his [ability to commit horrific acts] strengthened"

Is there a word that specifically refers to the measurement of someone’s ability to commit a horrific act without hesitation, mostly because they believe it to be necessary?

An example of a horrific act would be murdering an innocent for the purpose of a sacrificial rite because his cult demands it of him. Is there a word that describes his measure to do so?

Some words I’ve considered are "disregard" and "callousness" but neither seems to quite fit.