The Kradin were a large, humanoid species with muscular ridges around the face, and teeth on all sides of their mouth. According to the Vori, they were "beasts" who were capable of a variety of atrocities, rape, genocide, and killing Vori in their sleep.
The Vori furthermore claimed that the Kradin craved air supremacy and fired on any craft that didn't show Kradin colors. They were said to engage in a practice called neck-strapping, and left bodies upturned so they could not enter the Way After. They also claimed that their breath stank of muck, and perhaps figuratively, claimed that there was no heart thumping beneath their flesh and described them as motherless. (VOY: "Nemesis")
However, there was no independent evidence for the Kradin's vicious nature. In fact, when Commander Chakotay of the USS Voyager was kidnapped by the Vori, the Kradin aided in his rescue. (VOY: "Nemesis")