This is Socrates most controversial doctrine. He claimed that "No-one willingly does wrong." The first instinct is to deny it. But after consideration, provided there is nothing organically wrong with an individual, it is true. I submit no person decides to do something that will bring condemnation from friends, family and, even, strangers. Everything we do, even on the spur of the moment, has some rationale behind it, some kind of justification. Even the individual that thinks they might be on the edge of impropriety can accept their actions if they think no one will be harmed if they never learn of it.
There are many people we meet and hear of whose actions passeth all understanding; but I think, without excusing them for the consequences of their actions, we owe them at least a measure of compassion. Whether through ignorance, confusion or just plain stupidity, no thinking human wants to harm another for fun. There is no fun in it. The reason that many of us do the right thing is not, alas, because it is the right thing but because we want to avoid the punishment that will result later.
This is the basis for Mercy. The worse the wrong, the greater the actor will eventually pay, sometimes for the rest of their lives, with a misery that can never be washed away. Whatever short term punishment we can properly deliver will pale in a lifetime spoiled by a foolish act.
The most difficult problem comes with the pathological or the sociopath, the ones that have no way of knowing or caring for whatever wrong they do. Even worse, some of them delight in it. What should be done with such a one? Is it not cruel to confine them in solitary for the span of their lives? Should they be chemically or physically neutralized to stay alive but harm no others, rendered unaware of their surroundings? Would those remedies be no better, or worse, than putting them to death?