BrotherHostower said:
A 10+ year veteran sergeant is easily upstaged by a number of other ranks of space marine (terminators, captains, chaplains, librarians, techmarines, etc), so BS Ag would actually BE a good starting point to look at things.
Except that Brother-Sergeant Agamorr isn't just any veteran of a decade of warfare - he's over two centuries old, for one thing. He's capable enough to have been selected for Deathwatch service (which demonstrates a particular level of prowess above and beyond most Marines), and has since spent the last thirty years in the Deathwatch being a specialist alien-killer (having stayed on after the normal ten-year term).
He's a two-hundred-year-old Marine picked to be a part of the Astartes Special Forces unit who serve as the Chamber Militant of the Ordo Xenos. An average Battle Brother he is not.
And that's before you consider his equipment.