We know little about the Yahg, beyond their being brutally violent (by Council standards) and very intelligent (an outwardly strange combination). Reading the logs concerning his rise to power, I didn't find it hard to believe.
He seemed exceptional for his species, wanting to leave his world and find "what wonders awaited him" in space. He became the Broker's top agent, being both physically imposing and disturbingly intelligent. In time, the Broker realized his error in trusting this excessively brutal lieutenant and tried to correct it by having "Operative Kechlu" killed. It was too late. Kechlu, having learned all he needed to know, killed him and took over the job.
Kechlu was not a primitive brute. His predecessor paid with his life for underestimating him. I think the brief story we find in those logs paints a believable rise to power by an obvious prodigy. It's a shame we couldn't make an ally of him, having gone on about his brilliance. He was unfortunately born a Yagh, I guess. Shepard is not susceptible to Yahg bully-tactics, as Kechlu learned.
EDIT: I felt moved to read the Yahg wiki page, after making this post. These guys are the "best at everything race", excepting inter-species diplomacy. If it were marketable (which it is not), a Yahg would make a good protagonist for a single-player game. Smarter? Check. Stronger? Check. Faster? Check. Nearly impossible to hoodwink? Yep. These guys could produce some serious Mary Sues, if they got off Parnak. What are those crazy Salarians thinking? These guys are bigger than Krogan and as smart as Salarians. Yikes.
Honestly, I'd really like to see one of these guys in a future game, if they could find a "not lame" way of doing it. It seems they willingly and loyally follow a proven leader, so clearly we need a Yahg crew member, at some point.