This is my theory on why he did it .He planed it out. They say that there is now way to fully erase the mind. Izumi showed that this is tru in Gantz. He remembered parts of his missions and tried to get back in. Well what if Kurono set it up so that when he is free that he would find out about Gantz again. Maybe even though his computer he will read about it and remember bits and peaices. Now the big question is why. The vampires could strike at anytime and kill the gantzers. The only ones that have a hope for survival are really Nishi and Izumi the others would be shoked and probly get their asses handed to em on a silver platter not to mention maybe the vamps could interfear with the missions. So now maybe the vamps dont know the full deal of gantz and figure who better to recruit than a fromer Gantzer to hunt the thing down soi they eventually turn himm and try and use him to find gantz. However he wanted this and uses it to help kill off the vamps and their leaders or maybe even to finnaly destroy Gantz ending his game forever.