Ren was facing off against the supposed true enemy of his past incarnations the ORIGIN. Well, it wasn't really its true body but was using the body of a random stranger. Yet even though the person in front of him was a normal looking person Ren didn't let his guard down. When the ORIGIN said he would punish Ren the past incarnations within his soul started telling him to relinquish control of his body to one of them. Due to him being conscious it was harder to take hold of his body so they needed his permission.
Ren, of course, wouldn't allow such a thing to happen. Even though at the moment he was filled with rage and other emotions he normally wouldn't have, the excitement of battle still trumped all. The feeling of facing an opponent of equal strength was exciting enough, but to face an opponent that made you feel that you had no hope of winning was a thrill on a whole other level.
