As you consider the death penalty, it's important to make a distinction between the role of an individual and the role of government. If someone murdered your child, should you forgive them or should you fight to have them prosecuted? The answer is: both. The Bible commands us as individuals to forgive those who harm us. But as members of society, we're called to uphold standards and to protect others from harm to the extent we can. This is the first role of government, and government is given certain authority that individuals don't have.
For instance, when someone commits a crime, we as individuals do not have the authority to lock them up for a long period of time, or condemn them to death; in fact, it would be a crime for us to do so. But if we were members of a jury, we would not only have the authority but the responsibility to send them to prison - or to death, if that's what the law calls for. So, it's entirely consistent to forgive one who harms you and at the same time work to see that they are punished by the government.
If we want a just society, we must understand the distinctions in the roles we are called to fill.