:The source he is referring to is likely this: the article I discovered.  The reason I invited the wider view (mentioning that other political parties are involved in violence) is because it gives the reader a greater understanding of the elements at play.  Just as if one were to read about sea stars and there is a mention that they eat phytoplankton, mentioning that jellyfish also eat phytoplankton might be useful for greater understanding of the sea stars (are they in the same waters, do they compete for food).  To present this information as if CPI(M) members are singular in their violence is not a correct viewpoint if violence is ongoing between many parties.  In another example, if we were to present a fistfight between a man in a red shirt and a man in a white shirt, when they both were arrested for fighting... If we reported that as "The man in the red shirt was striking the man in the white shirt repeatedly until eventually he was arrested." is not neutral because it gives undue weight to only one portion of a greater conflict, not informing the reader of all the information they could otherwise have.

As to your other question.  "What wiki policy prohibits use of only one source?" if the source is reliable for the type of information the source is providing here, then it may not be correct for it to be prohibited.  If other reliable sources disagree about the existence of Harmath, then we'd have a different avenue to go.  However, this is the Wikipedia policy I am currently the most concerned with.  If Harmath is truly only supported by a single source, mentioning them unequivocally as a militant arm of the party based on that one source might be undue weight.  Saying "One source claimed that an organization called Harmath is a militant arm..." might be better, but we must take care not to use that as a gateway to simply reprint the article here.  If there is but one source, the weight that this should be given is likely minimal.  Does that properly speak to my viewpoints?  72.192.46.9