'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?
I think your question contains an implication that one practicing metta will be inclined to do nothing for the person they feel friendly towards. This is not the case at all. A person with metta is inclined towards the benefit of others:


  For among these, loving-kindness is the way to purity for one who has
  much ill will, compassion is that for one who has much cruelty, gladness is that
  for one who has much aversion (boredom), and equanimity is that for one who
  has much greed. Also attention given to beings is only fourfold, that is to say, as
  bringing welfare, as removing suffering, as being glad at their success, and as
  unconcern, [that is to say, impartial neutrality]. And one abiding in the
  measureless states should practice loving-kindness and the rest like a mother
  with four sons, namely, a child, an invalid, one in the flush of youth, and one
  busy with his own affairs; for she wants the child to grow up, wants the invalid
  to get well, wants the one in the flush of youth to enjoy for long the benefits of
  youth, and is not at all bothered about the one who is busy with his own affairs.
  That is why the measureless states are only four as “due to paths to purity and
  other sets of four.”
  
  One who wants to develop these four should practice them towards beings
  first as the promotion of the aspect of welfare—and loving-kindness has the
  promotion of the aspect of welfare as its characteristic; and next, on seeing or
  hearing or judging that beings whose welfare has been thus wished for are at
  the mercy of suffering, they should be practiced as the promotion of the aspect of
  the removal of suffering—and compassion has the promotion of the aspect of
  the removal of suffering as its characteristic; and then, on seeing the success of
  those whose welfare has been wished for and the removal of whose suffering
  has been wished for, they should be practiced as being glad—and gladness has
  the act of gladdening as its characteristic; but after that there is nothing to be
  done and so they should be practiced as the neutral aspect, in other words, the
  state of an onlooker—and equanimity has the promotion of the aspect of
  neutrality as its characteristic; therefore, since their respective aims are the aspect
  of welfare, etc., their order should be understood to correspond, with loving-
  kindness stated first, then compassion, gladness and equanimity.
  
  -- Vism IX.108-9 (Nyanamoli, trans)


Clearly a person with a mind of metta is more likely to work to help those most in need; not just to end the suffering of victims, but to end the evil of perpetrators that is the cause of suffering for both.

Do not the wicked "deserve" suffering for the suffering they've caused?

Certainly, this seems reasonable, but does not relate to one's own state of mind. Anger is still evil even when it is based on true assessment of the situation; two wrongs don't make a right.

When we generate metta for the "wicked", how do we thus distance ourselves from the horrible actions of our metta-recipients? 

We do it by beginning with lovable beings, then slowly working our way up to neutral, then finally hostile individuals. Any time we find our state of friendliness slipping, we revert back to an easier object.


  If resentment arises in him when he applies his mind to a hostile person
  because he remembers wrongs done by that person, he should get rid of the
  resentment by entering repeatedly into loving-kindness [jhāna] towards any of
  the first-mentioned persons and then, after he has emerged each time, directing
  loving-kindness towards that person.
  
  -- Vism IX.14 (Nyanamoli, trans)


There is much more detailed info on dealing with anger in chapter nine of the Visuddhimagga, along with quotes from the Buddha.

Is there any concept of "condemnation with metta"?

I think you are under the impression that metta is a mind state to be dwelt in constantly. Buddhism doesn't espouse such a view; constant attention to metta will lead to rebirth in the brahma realms, not enlightenment. No, it is not possible to condemn someone while absorbed in the practice of metta meditation, but it is possible for a person free from anger, who often practices metta, to do so. Metta may in fact lead someone to condemn others, thinking that the condemnation will help them overcome their evil deeds, or help keep others from falling prey to their evil deeds (which would probably be karuna).


  ‘Then Sunakkhatta came to me, saluted me, and sat down to one side. I said to him: “You foolish man, do you claim to be a follower of the Sakyan?” “Lord, what do you mean by this question?” “Sunakkhatta, did you not go to see Kaḷāramuṭṭhaka and ask him a question he could not answer, and did he not thereupon show signs of anger, rage and petulance? And did you not think: ‘I might cause this real Arahant ascetic offence. I don’t want anything to happen that would be to my lasting harm and misfortune’?” “I did, Lord. Does the Blessed Lord begrudge others their Arahantship?” “I do not begrudge others their Arahantship, you foolish man. It is only in you that this evil view has arisen. Cast it aside lest it should be to your harm and sorrow for a long time! This naked ascetic Kaḷāramuṭṭhaka, whom you regard as a true Arahant, will before long be living clothed and married, subsisting on boiled rice and sour milk. He will go beyond all the shrines of Vesālī, and will die having entirely lost his reputation.” And indeed all this came about.
  
  -- DN 24 (Walshe, trans)