Hmmmmm, I'm thinking you're onto something. The only thing that I'd be concerned with is the armor advantage a Spartan has over a Scout Marine. I'm sure you know but just in case other don't, a scout marine only wears armor on his upper body and head. The reason for this is because they often undertake the stealth, reconnaissance operations for the Legion and carrying bulky armor would make that job difficult obviously.
The Astartes have their own versions of assassins though. Marines who exhibit a violent and murderous streak get secluded and placed into specialized squads called Moritats. They are essentially space marine assassins trained in the unique ways of stealth/silent killing each legion has.