The Klingons aren't sadists but they do not hesitate to use force when it is felt to be necessary. As these are the individuals other space faring races typically encounter, it is these imposing Warrior types that give the first impression of what Klingons are.
Despite what other races may believe only a tiny portion of their planetary populations are selected to be warriors and these are typically quite large, muscular and imposing in their demeanour, more so than their planetbound, civilian or service industry brethren.
PRIME DIRECTIVE RPG MODERN SKIN
Klingons are humanoids very similar to terrestrial humans excepting that they have a larger, ridged cranial structure, much darker skin and typically predatory teeth and nails that are almost talon like in the way they grow.
It is tradition for sons of warriors to follow in their father's footsteps, although such heritage will not make their passage to the ranks any easier.
PRIME DIRECTIVE RPG MODERN SERIES
Klingon youths must take special training and pass a series of tests to become warriors, and while there are no formal barriers to sons of civilians becoming warriors, few who grow up in a civilian environment show any interest. Virtually all Klingons encountered in neutral space and on away missions undertaken on non-Klingon worlds, are of the Warrior class. There are two primary Klingon social classes: warriors and civilians. Klingons are generally regarded as a brutal warrior race, but this characterization is not entirely fair or accurate. The Klingons have alliances with the neighboring Lyrans (a race they have fought in the past) and the Romulans. Two minor powers, the Tholians, who occupy stolen Klingon territory along the galactic edge, and the WYNs (a multi-race collective), are also on the Klingon borders. The Klingons have three major enemies: the Kzintis and Hydrans, with whom many wars have been fought, and the Federation, which has not often been an active military enemy but which threatens the Empire with economic domination. Any attempt to match the Federation standard of living would require disbanding the military forces, leaving the Empire vulnerable to a host of enemies. The Klingon Empire is a poor one, and it can only match the Federation in military power at the cost of a lower standard of living for its people. When the Kings disappeared (legend has it they left to avoid the death of the Galaxy, since stars in the core are already falling into a gigantic black hole), the Klingons quickly picked up the pieces and forged an Empire of their own using skills and technologies left behind by their previous masters. The Klingon Empire arose from the ruins of a previous empire of a race known only as "The Old Kings". The Klingons had been a subject race of the old Kings and were used primarily in their starship crews. Klingons are a major fixture in all eras of the Star Trek universe, being one of the major enemies of the Federation early on only to become an oftentimes uneasy partner for the Federation.