Legion of the Damned by William C. Dietz
Far in the future, condemned criminals and the fatally ill have two choices: to die, or to join the Legion of the Damned, an elite fighting force made up of human legionnaires and cyborg soldiers. If they enlist, their bodies will be destroyed and their minds implanted in gigantic armored killing machines, programmed to fight for the Human Empire. As trouble brews among the Confederacy of Sentient Beings, the aggressive Hudathan aliens are turning their attentions towards Earth. Ruled by an insane emperor and a corrupt government, it's up to the Legion to save the planet and the Empire from total destruction.