Being an artificially being, Genis-Vell was not really ready for the tremendous power he inherited from his father. Case in point - while trying to master his Cosmic Awareness, Genis was overwhelmed by the infinite information, and went mad.
The now-insane Genis encounters a branch of the Kree Navy, and forcibly joins their army, adopting a version of their uniform. Genis wears this uniform as he goes on to destroy the universe, and then recreate it to give himself the perfect life that he wanted. Ultimately his plan failed, fueling his insanity and desire for self-destruction, and it took an intervention from his family to restore Genis' sanity.
