It's too easy to say "well finding his wife in bed with another man made him snap, let's not be too hard on him and only give him a few months" and forget that criminals destroy the very base of society and the very trust people have in each other.
We already do this. Circumstances, character, history, and motive will affect sentencing and influence juries, and this is not even considered a bad thing (for the most part). It's a part of the system.
If you start showing compassion to those that don't have it and see it as a sign of weakness then they will exploit that and you're left at the mercy of people who apply that line of reasoning everywhere.
See what as a sign of weakness?
Also: Ostensibly, we
do show compassion toward criminals. That's why we disallow "cruel and unusual punishment" and don't just execute people for everything.
You're also making a really bad slippery-slope argument. You might as well say that if we start letting people publish documents criticizing the government, people will exploit that to form terrorist cells. There is no reasoning. It is a non sequitur.
I've also already established a couple lines of reasoning to show that greater compassion for criminals and greater understanding of their behavior can
reduce crime, not to mention it's obviously the only decent way to actually rehabilitate those who can be rehabilitated (both before and after they commit an offense).
Thread moving too fast... moving the edit of my last post here to be sure it isn't missed.
Edit: You added the line about compassion... I actually believe that it's extremely rare for a person not to have compassion, which is exactly WHY this tendency to reclassify human beings as something else, such as criminal, exists. If we weren't able to do this, we wouldn't be able to do horrible things to each other.
However, it's often difficult for people to have compassion for those unlike them, in other social circumstances or groups, and so forth, and there are tendencies when it comes to the more "unthinkable" crimes to demonize the perpetrators.