Planet of the Damned, by Harry Harrison. Part I.

Dis was a harsh, inhospitable, dangerous place and the Magter made it worse. They might have been human once—but they were something else now.

The Magter had only one desire—Kill! Kill everything, themselves, their planet, the universe if they could—Brion Brandd was sent in at the eleventh hour. His mission was to save Dis, but it looked as though he was going to preside over its annihilation. Space derring Do. Rip roaring action. Evil Aliens. A bewildered hero. Lots of fun.