What does it take to forgive your killers, of a slow painful horrific death?

Imagine people just say something bad to you and your revenge defensive mode kicks in. Imagine a slap in the face or a punch or something worse. But a crucifixion and a stoning? What does it take, or what is needed for people under such raw realities to be able to forgive their executioners and those involved.

Examples:

Jesus Christ: Then said Jesus, "Father, forgive them; for they know not what they do." Luke 23:34

Stephen: Then he fell on his knees and cried out, "Lord, do not hold this sin against them." When he had said this, he fell asleep. Acts 7:60