Not too sure of their background, but I do remember them being pricks in the Salamanders books, stealing a shipload of Heresy era armour, and there's the incident on Armageddon when they bombard a refugee camp just to kill a few orks. They certainly have earned their title.
I don't think it's so much that they hate each other but rather self interest. If betraying another marine could advance their position or save themselves they'll do it. In the salamander books they worked ok with the salamanders up to a point.
But everything else to chalked up to them being borderline censured.
When a Salamander captain punches another commander it says allot about them. The salamanders are known to be the nicest chapter willing to respect all humans, just because that's their way.
Yeah but I just think it's really odd. They are so completely different to all other loyalist Chapters.
Looking at the evidence from White Dwarf 249, we know that they existed in the 32nd millenium so it's possible they are a 2nd founding Chapter from an unknown source so lets look at the possible sources of their geneseed:
Starting with the Loyalists:
The Blood Angels and their successors are basically portrayed as similar to the pre Angron War Hounds (or in the case of the Flesh Tearers almost identical to the butchers nails World Eaters). The MM's don't operate in this way and there is no evidence they suffer from the Black Rage or Red Thirst so we can rule out the Blood Angels.
The Ultramarines and their successors are a bit of a mix but all seem to be honourable and not self serving. We know the Mortifactors are one of the Chapters which has changed the most from their Ultramarines source but even so they are still honourable and not back stabbing. Equally the Doom Eagles are changed in their outlook but equally are honourable and not back stabbing. It's possible the MMs are descended from the Ultramarines but unlikely in my opinion.
The Space Wolves are loyal to each other like the wolves they think they are and there is no evidence in the stories about the MMs having fangs so I think we can rule the Wolves out.
The Salamanders? Well it's interesting we got to see a couple of nasty ones in one of the Sallies novels and these nasty ones did seem to bear a similarity to the MM but these seemed to be the bad apples as it were as the Sallies as a whole did not share this bad attitude.
The Raven Guard and their successors are a very interesting one for me. We know that Corax expelled several Nomad Predation fleets when he took over the XIX Legion. It's possible that like the Carcharodon Astra, the Marines Malevolent were another of theese. After all Corax didn't like their attitudes towards humanity and the modus operandi in warfare. It's not such a huge leap to imagine that one of the nomad predation fleets would be seen as an outcast by the Imperium and thus would resort to scavenging and their attitudes towards humanity would morph into a survival of the fittest mentality and they would turn on each other .
The White Scars are another interesting one simply as we don't know enough about them but from what we do know from Scars and Brotherhood of the Storm I just don't see the MMs as a successor Chapter of Jaghati.
Iron Hands - the MMs don't have the technical aptitude so I think this rules out the IHs and their successors.
Finally I just don't see the MMs as being a successor of the Imperial Fists. The Sons of Dorn simply have a radically different mindset.
The Traitor Legions:
Emperors Children - no similarities at all so I can't see them being a loyalist remnant.
Iron Warriors - the MMs don't seem to have the technical skills (as we see they don't seem to be able to repair their broken armour) so this seemingly would rule out the Iron Warriors.
Night Lords - Another prime contender. We've seen the pre heresy attitudes of the Night Lords towards both each other and humanity in general. We've never been given a description of a MM Marine without his helmet so we can't be certain if they have the pale skin etc of the Night Lords but the MMs seem to share their modus operandi.
World Eaters - These had me thinking. Pre butcher's nails could be a possibility but the likelihood of a loyalist remnant of the WE who don't have the nails (since there is no indication of the MMs having any form of the nails) is very small so I've had to rule them out.
Death Guard - It's possible that the MMs are a loyalist remnant of the DG. The DG cared little for collateral damage and got the job done using whatever means necessary. For me they are a maybe.
Thousand Sons - Absolutely not - no indication of psykers in the MMs.
Sons of Horus - Like the Death Guard there is a possibility the MMs are a loyalist remnant but their modus operandi isn't that similar. Again it's a maybe but less than the DG in my opinion.
Word Bearers - Absolutely not - we know that all loyalists were purged.
Alpha Legion - Who knows?? It's possible I suppose but the MMs modus operandi is very different to what little information is known about the AL. Then again if loyalist AL were trying to hide within the Imperium post Heresy then what better way to hide than to take on a mantle different to that of the original Legion. Over time their attitudes may have changed sufficiently to become that which they initially were only pretending to be. As with anything connected to the AL it's a maybe but we'll never know.
Going simply from the MM's modus operandi I can only see them being from 1 of 4 sources with the most likely (in my opinion) first :
Night Lords loyalists - the MMs share the NLs way of thinking.
Raven Guard via the Nomad Predation Fleet route
Death Guard loyalists
Alpha Legion loyalists