Not every xenos-related crisis demands the full deployment of a Deathwatch Kill-team, but many seemingly inconsequential incidents can be harbingers of a greater threat that would be foolhardy to ignore. Under these kinds of circumstances a Watch Captain may elect to send a single Space Marine, a specially trained "Kill-marine" skilled in solo operations, to investigate and exterminate where possible or to call in backup where it is truly needed.
Kill-marines are naturally denied the reliable supporting firepower of their Battle-Brothers and fall back on the stealth and infiltration techniques they first mastered as Scout/Initiates to perform their duties. Scout Sergeants often make superlative Deathwatch Kill-marines with little additional training but these specialists are drawn from all ranks of the Deathwatch as needed. A certain independence of thought and great strength of spirit are in many ways more important than exceptional stealth skills for a Kill-marine. A Kill-marine must possess the right temperament to operate for long periods cut off from their kind and from the sacred touchstones of their Chapter.
Space Marines ordinarily live a cloistered existence in their Fortress-Monasteries. Their contact with the outside world is normally limited to fighting wars in it and their interactions with humans only come in chance encounters on the battlefield. By contrast a Kill-marine has to interact frequently with the existing authorities and power structures of the Imperium, often acting as a direct representative of the Deathwatch before Planetary Governors, war councils and ruling bodies of Adepts.
Kill-marines spend time living alongside those they must ultimately defend, sharing their trials and seeing the world through their eyes. In their missions a Kill-marine will make allies and enemies of many that they meet, and hear tales that would never reach a Watch Fortress. He might become involved in events that ultimately have little to do with the Deathwatch directly but rebound to the greater security of the Imperium. In this way the remote Deathwatch maintains a limited connection to the great masses it is charged to protect even as it stands vigil in the darkest gulfs surrounding mankind.
In practical terms the greatest step a Kill-marine must take is to secure travel between the stars as Deathwatch vessels can rarely be spared for their direct deployment. Interstellar ships are such rare and precious things that even the word of a Space Marine will not turn one from its course. More militant Battle-Brothers have sometimes opted for the expedient of capturing a pirate vessel and forcing its crew to do their will, exterminating their unwilling hosts once they reach journey’s end. However many less drastic opportunities exist for a Kill-marine that is prepared to think more broadly. He might travel aboard Imperial Navy warships and Rogue Trader vessels for example, or join the retinue of an Inquisitor for a time if it will serve his purposes. To do this the Kill-marine may have to use his natural humility to ask, and not demand, his passage by offering his assistance in trade for reaching his destination.
Imperial authorities will usually welcome the opportunity to gain access the knowledge and experience of a Space Marine for their own purposes in some vexing matter. The singular skills a Kill-marine can bring to bear are liable to open whole realms of possibility that were previously out of reach to a Rogue Trader Captain or an Inquisitor. A previously unknown and potent player entering the game can be invaluable for their schemes. The guarantees of transportation to an undisclosed location that a Kill-marine may ask for are a small price to pay to secure his help.
More cynical commentators have noted that the portrayal of the Kill-marine as some kind of diplomatic emissary is disingenuous at best and an outright lie at worst. A Kill-marine is still a Space Marine, a super-humanly fast and strong warrior trained in the very highest arts of war. Some accuse Kill-marines of being spies and assassins operating at the behest of the Watch Commander, independent and unregulated agents of a shadowy organization with a reputation for exterminating entire worlds. The coming of a Kill-marine may be treated with great alarm by some, an unsubtle reminder that the harbingers of death are near. 
Kill-marines do often forego power armour and use non-standard weaponry of a somewhat stealthier demeanour than the sacred boltgun and holy chainsword. They maintain that this practice is born of practicality when they are commonly moving through environments designed for an unarmoured man, and that employing more easily concealed weaponry only serves to soften their otherwise intimidating and warlike aspect. It is to be expected that such peerless warriors would adapt to their circumstances by honing their stealth and unarmed combat skills to exceptional levels so that they can slay quickly and quietly in case of trouble. 
However such paranoid accusations may have some foundation in reality. At times Kill-marines have turned on those apparently giving them aid, even unto slaying entire households from within on the pretext of uncovering corruption in their ranks. How often these events are rooted in coincidence and spontaneous action as opposed to the carefully considered orders of the Watch Commander is impossible to say. The responses of the Deathwatch to those brave enough to level such charges are simply that those who have no truck with the alien have nothing to fear.
It is true that Kill-marines are often sent on missions obscure to their Battle-Brothers in the Deathwatch and even to themselves. The Librarium and the Apothecarion can layer mental blocks and hypno-conditioning into a Space Marine’s mind to make it receptacle for the most sensitive knowledge. Secret orders or communiqués that a Watch Captain will not entrust to Astropathic communication may be buried deep in a Kill-marine’s subconscious for him to transport to another Watch Fortress. A Kill-marine may carry, knowingly or unknowingly, relics or other items of key importance for delivery into the hands of specified individual. They may be assigned to guard an individual while having pre-programmed assassination protocols in place to eliminate them if the call arises. Some whisper that these enigmatic figures have even dealt directly with aliens, a feat that would surely require extreme Hypno-indoctrination for a Deathwatch Space Marine to achieve.
|Command||Watch Master • Watch Captain • Chaplain • Librarian (Epistolary) • Forge Master • Keeper|
|Elites||Veteran • Vanguard Veteran • Terminator • Champion • Dreadnought • Venerable Dreadnought|
|Vehicles||Rhino • Razorback • Land Raider • Land Raider Redeemer • Land Raider Crusader|
|Aircraft||Corvus Blackstar • Drop Pod|
|Fleet||Kill-Ship • Strike Cruiser|