Ruthless could be an embittered mage who hated the circle, hated templars and felt resentful of having to do the extreme (joining the wardens) just to get out of it. He could hate everyone except himself (and Morrigan, hell maybe even her too). Since she fears/hates mages so much, if I were going that route I'd sacrifice Isolde to Jowan's blood ritual, seeing in her a representation of all that the 'mundanes' view mages, but then Jowan wouldn't get away.
Ruthless could be an angry Cousland who hates, and looks forward to shoving the blade into Howe's belly--or back--with relish. Be progressive about mages (or just not care Morrigan is one) and resents the wardens, and the fact that Duncan was a bit of a ****** in that origin, only agreeing to help if your dad (who's bleeding out on the stone floor of the keep) gives his consent to your Joining. He might have a bit of 'hero worship' for Loghain seeing as he would be the one most educated in current Ferelden events and know of the recent history of the country, and Loghain's participation in its liberation.
Ruthless could be a xenophobic Dalish who resented Duncan smashing the mirror that could have healed him (I know it can't but your PC may not see it that way) and angry that he took advantage of the situation. He might see this as 'someone with power abusing it to his advantage' (Several of the dialogue lines have you asking him why he doesn't just 'give you the cure'). He might respect mages since his Keeper is one (as was his father) and find a similar mind set in Morrigan.
Ruthless could be a City Elf who had to scrabble and take for every scrap to survive (not so much with other elves but humans) and be resentful, especially of nobles like Vaughan who think they own every one. He would have less reason to romance Morrigan maybe with this mindset, but he may figure, she has power and she knows how to use it to get what she wants, and he'd like to learn more? He may not resent the wardens, but wouldn't have any love for humans (esp the nobility).
Ruthless could be an ambitious Dwarf Noble who actively plotted the demise of Trian and falls for Bhelen's trap. He romances Morrigan as just another step to power--perhaps initially thinking to get her to love him and maybe falling into his own trap. Sense a pattern here? Maybe he's not as clever as he thinks he is?
Ruthless could be a Dwarf Commoner who (like the City Elf) had to scrabble for every scrap just to survive, only with more urgency since, unlike the CE, he really is alone except for his sister, Rica and maybe Leske (his Carta partner).
I can't really play ******* characters, and the game kind of pigeon-holes you to a 'good' path but there are some ruthless steps you can take that allow for a more '*******' approach.
For a paragon, I see Cousland as best, since they could be played as "noble" and wanting to save Ferelden from the darkspawn. Though a mage works well too in this capacity.