To begin with, contrast several translations of Qur'an 24:19:
Truly, those who love that indecency be spread about those who believed, they will have a painful punishment in the present and in the world to come. And God knows and you know not. -- Dr. Laleh Bakhtiar
Verily, those who love to see slander (or scandal) broadcast amongst the believers, for them is grievous woe in this world and the Hereafter; Allah knows, but you know not. -- Hamid S. Aziz
Those who love to broadcast such slanders among the believers will have a painful punishment in this life and in the Hereafter. Allah knows and you do not know. -- Farook Malik
Indeed, those who like that (should) spread the immorality among those who believe, for them (is) a punishment painful in the world and the Hereafter. And Allah knows, while you (do) not know. -- Literal
(Word by Word)
How to make sense of these? Here are some tafseer:
Islam has taken as serious a view of the spreading and circulation of false accusations as that of crimes against chastity. It has condemned and prescribed punishment for both the crimes---for scandalmongering, even a severer punishment since it is calculated to produce more grievous consequences as far as the prevalence of sexual immorality in a community is concerned than they stray acts of sexual misconduct. If scandalmongering is allowed to be indulged in unchecked in a community, it will come to lose all sense of horror and abhorrence at the commission of immoral acts with the result that immorality will become rampant and a sense of pessimism with regard to its future will begin to prevail in the community, thus shaking its whole moral foundation. -- The Holy Qur'an, Arabic Text with English Translation and Short Commentary, Malik Ghulam Farid
The direct interpretation of the verse, in the context in which it occurs, is this: Those who cast aspersions, propagate evil, publicize it and bring Islamic morality into disrepute deserve punishment. The words in the text, however, comprehend all the various forms that can be employed for the propagation of evil. These include actual setting up of brothels, production of erotic stories, songs, paintings, plays and dramas as well as all kinds of mixed gatherings at clubs and hotels, which induce the people to immoralities. The Quran holds all those who resort to such things as criminals, who deserve punishment not only in the Hereafter but in this world as well. Accordingly, it is the duty of an Islamic government to put an end to all such means of propagating immorality. Its penal law must hold all those acts as cognizable offenses which the Quran mentions as crimes against public morality and declares the offenders punishable. -- IslamicStudies.info
Vamping and circulating false rumours is one of the main branches of conspirators' psychological warfare. When enemy cannot bring damage to the other party face to face, it resorts to spreading rumour. ...
In the concerning verses, according to the famous occasions of revelation, for weakening and marring the reputation of the Messenger of Allah (S), some hypocrites fabricated and circulated the most unfair rumour. ...
People must be aware that when they hear a rumour, they must bury it immediately; otherwise we have made the enemy happy and successful. Moreover, we will be imposed with painful punishment of the world and the Hereafter, which has been mentioned in the above verses. -- al-Islam.org
Those who spread news and evil rumors and propagate wickedness in the Muslim Community, deserve punishment and not encouragement. -- EnglishTafsir.com
In this case, the ayah was interpreted as meaning "spreading evil is to be punished", whether it be through rumours or spreading immorality in general.
As far as I know, it is acceptable to warn someone about a person who truly commits evil (Islam Q&A even goes so far as to say its waajib [obligatory]). However, keep in mind the exceptionally high standard of evidence required:
In order to prevent such a great injustice from occurring, Islam has commanded the Muslims not to pay attention to just any piece of news or information that comes to them and not to accept just anything that anyone says to them. -- al-Islam.org
And keep in mind the necessity of warning others:
Muslims should be careful and verify whether a piece of news is true before circulating it. ... to hide people's faults is more beneficial than defamation even though the information at hand is true and valid. The Prophet Muhammad pbuh used not to mention the name of someone he would criticise. -- On rumour mongering, fatwa
Justice is not guaranteed in this world. Indeed, we see all sorts of injustices taking place in the news. It's probably best to leave justice to law enforcement and Allah's perfect judgement.
However, Islam is a religion of forgiveness:
Whoever suffers an injury and forgives (the person responsible), God will raise his status to a higher degree and remove one of his sins (source)
I also think it's dangerous to brand people as non-Muslims. For example:
Sins committed by a Muslim, be they deadly or trivial, do not render a Muslim a non-believer. -- The Response
People go around declaring Muslims as non-Muslims all the time (takfir), but it's a major sin if incorrect.
Takfir or the condemnation of a Muslim by another Muslim as a kafir is strictly prohibited in the Quran, the Hadith, and the writings of many eminent Muslim authorities. -- On the prohibition of takfir, fatwa
