Justifiable violence in Buddhism? Does Buddhism preach absolute non-violence? Or is there any instance where Buddha had thought that violence can become a necessity under certain situations?
No, householder Somanna, the Buddha and all wise approve that one thing should be killed, and only that:

Having killed what
    do you sleep in ease?
Having killed what
    do you not grieve?
Of the slaying
of what one thing
    does Gotama approve?

[The Buddha:]
Having killed anger
    you sleep in ease.
Having killed anger
    you do not grieve.
The noble ones praise
the slaying of anger
    — with its honeyed crest
    &amp; poison root —
for having killed it
    you do not grieve.

Ghatva Sutta

Without need to have any doubt, if hearing or seeing that someone approves harming, encourages to such by signs and words or even performing such, that such person is not even near related to the Buddha, wise and their following, but harmful thieves in this world.

Those who have doubts, regard them as not having found refuge in the tripple Gems jet, how much ever they use the Gems, even tell that they are Buddhist, knowing, and follower.

May one be careful and blessed to never assosiate with fools and bad friend.

As already quoted, not all are capable in getting the message.

The Simile of the Saw, is possible the most famous collection on it.

Not for any purpose:


  "What do you think Dhanañjani? There is the case where a certain person, for the sake of his mother &amp; father...for the sake of his wife &amp; children ... his slaves &amp; workers ... his friends &amp; companions ... his kinsmen &amp; relatives ... his guests ... his departed ancestors ... the devatas ... the king, ...There is the case where a certain person, for the sake of refreshing &amp; nourishing his body,..., does what is unrighteous, does what is discordant. Then, because of his unrighteous, discordant behavior, hell-wardens drag him off to hell. Would he gain anything by saying, 'I did what is unrighteous, what is discordant, for the sake of my mother &amp; father. Don't [throw] me into hell, hell-wardens!' Or would his mother &amp; father gain anything for him by saying, 'He did what is unrighteous, what is discordant, for our sake. Don't [throw] him into hell, hell-wardens!'?"
  
  "No, master Sariputta. Even right while he was wailing, they'd cast him into hell."...


Harmlessness is one of the three factors of right effort,following right view.

What ever might appear as harmfull, like rebuking fools, there by the words, right speech,  of compassion ends what ever conductive support can be given for one who acts (by thought, words and speech) for his longtime disadvantage.

(Note: this is not given for trade, exchange, stacks or entertainment, but only as means to escape this binding wheel)