Ok look! The key to understanding Nishi is to re-read that section where kurono has beaten the crap out of the onion alien daddy and Nishi captures him with his Y gun. Note! Nishi does not imediately send the alien away he gives kurono the chance to take the points, which he declines, before he captures him.
What can we read from this?
1 that Nishi is not a kill theif
2 that Nishi wants to enlist smart skilled players to create a team. perhaps like the team he may have been in before? with Izumi? or somone who knew izumi? Yes thats right, i am not necessarily assuming that Nishi directly knew Izumi but instead, i am assuming that there was a culture within gantz of living players who knew Izumi before he left that also knew Nishi (when he arrived).
I'm pretty sure he knows Izumi. Remember in the subway, when they were talking. Nishi said something like; 'Don't you remember me?'.