So, I re-routed the power to McCarran and the Strip like the dude told me to, but I made the mistake of activating the weapon and killing the NCR soldiers that were nearby. Now when I try to go back and tell the guy that it's done, he won't talk to me. Whenever I leave and come back, the NCR soldiers try to attack me.

I left the area and did some NCR quests to raise my reputation. I've even talked to the Ambassador and he "pardoned" me, but those guys at Helios One still want to kill me.

It doesn't even seem to help if I wear NCR armor, they still attack me.

Any ideas?

i′ve got the same problem.
i re-routed the power to the wasteland and after that the security...
he won′t talk to me. just killed all of the NCR Member to get outside and did a lot of quests but there is no chance to talk to him.

no chance to complete the quest?

Happend to me as well.
Just loaded an earlier save and rerouted the power, but didn't touch the weapon. After that, it was all ok