As scary as a sadistic Astartes who can literally hide in your shadow? Now now, calm your horses

That's why I said almost you forgot the immunity to pain and general insanity factors. Anyways when dealing with normal humans Marcus and Landon seem to be the most inhuman in the group while even the admech personnel atleast act sane Marcus is prone to extreme rage when off his meds and has forgotten the definition of mercy on more than one occasion.

Yeah our guys are far more inhumane than Theta-Xi and our dear Magos, lol. I guess they will get along.... Eventually :X

I think it would more or less be them nodding to one another in some form of respect between monsters

If there was a movie or story about it, it would be called "The Beast and the Other Beast"

No we can think of a better one give me a second.
*thinks*
"Monsters: Comrades in Brutality"
Sounds like an action movie from the perspective of the bad guys. Maybe just "Beasts" or "Counting Kills" just brain storming really.

Bromance: 40k style

Landon:"So....you're still alive?"
Marcus:"Yep"
Landon:"Good"
*they never speak to each other again*
Or at least that's how I envision it

*Tyesha stumbles upon this*
"AWWWWWWWW, you guys are so cute! Just hug already!"
And that's how I envision it

*Marcus' immediate response*
"Go frak yourself Tyesha."