Having just picked up a copy of Rites of Batle, I found myself quite interested in the place Kill-Marines play in the deathwatch. i know they're astartes that can function on their own and all that, skilled in bringing down the hated xenos on their own and all that. but what is their actual function? why would you send just one marine when Astartes are more or less made to function in squads? What are their other uses? and how do they interact with traditional kill teams?