'Metta' versus 'passive acceptance for the wicked'?

I've often seen the question asked: "How can we have metta for murderers, terrorists, criminals, etc.?" Though there are many good answers to these questions, I've noticed that few of them tend to highlight the difference between 'having metta for something or someone', versus 'condoning, endorsing, or even simply tolerating their actions'. Most answers stress that the wicked be viewed as individuals, who -- if they were truly happy -- would not do such terrible things. Therefore, we should wish that they attain that very true happiness. But that doesn't seem to change the fact that by generating metta towards these people, I often feel as though I am passively accepting their terrible deeds, which caused tremendous suffering to others. Do not the wicked "deserve" suffering for the suffering they've caused? When we generate metta for the "wicked", how do we thus distance ourselves from the horrible actions of our metta-recipients? How do we have unconditional loving-kindness for them without taking on their negative qualities and actions that make the metta so difficult to generate in the first-place?