This question came up on a facebook group with GW employees on it some time ago now. Supposedly the Space Marine fluff has always been super human freaks - that was what the gene enhancements did.
And they never stopped being recruited from worlds where warriors were born from the conditions they grew up in. Caliban was such a world before it got torn apart and reduced to a meteorite.
Then the monastic bit. I think they just pushed the crusading knights thing a bit more in both directions. As in, they took both the monastic bit some were and the rampaging homicidal butchers others were and put them together.
So whilst on their ships and between battles they are monk like. When in battle they unleash hell and have no limits to the destruction they will reek upon their enemies.
Talking of how to they fit in suits of armour.
The old rogue trader marines. I mean, I only caught the tail end of those in the early 90's. And I have little nostalgic fondness for them. But how in gods name was a super human anything meant to fit in that armour?