Question: In Fallout:NV, I initially was going against the NCR early on, but switched allegiances to see how their storyline played out.

All NCR troops accepted me as OK except for HELIOS ONE, where I had diverted power to all neighboring regions. They would attempt to kill me on sight, so I killed them all.

The problem is that I still have a quest open for requesting support from them....any way to complete this quest? I would have expected that it would fail once all the troops were dead...

The actual quest is 'Restoring Hope.'

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> Go to HELIOS One to ask about Forlorn Hope supplies.
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Answer: 
You should find Lt. Haggerty in HELIOS One to get information on the supplies you are tracking. If you have killed her, the quest is still solvable by finding the supplies without the tracer. They are directly north of the northeast corner of the Southern Nevada Wind Farm. The quest should update then to your next goal.

If that doesn't work, check where the quest marker on your compass is pointing to. Perhaps you've forgotten to do something in the quest.
For anyone coming to this currently, make a bee line from helios one to forlorn hope, you'll see ncr agents dead surrounding a case.