[Suicide Junkie]

Another way to protect the neutral races would be to give them special racial techs.

Then if you wipe them out, the techs are lost, but if you keep them alive, you can capture their ships & use the flux capacitor cannon that they put on.

PS: the flux cannon would be somthing like the temporal shift-em-to-beginning-of-time device, but would smear the ship out over all time instead.