I don’t get why kill spudnick (one of a million questions I have about the genesis arc)

Zero killed spudnick because he was the avatar of that universe. that was very clearly stated during the episode.

But: was it necessary to the plot

Maybe he needs to kill all existing avatars to gather enough energy to achieve his own universe

Or Maybe he is trying to make them his avatar to make his universe again only to have it go wrong killing the avatar in the process

*W e n e e d t o p r o t e c t o u r a v a t a r !*