Poltergeist at the Vehicle Graveyard

[Humpelfred]

Hi, on my second visit at the Vehicle Graveyard (the one in the Garbage, infested by rats) i encountered a scientist (plus several dead ones with heavy key cards in their inventory). He was sitting in a bus, asking you to kill the poltergeist which hides in the same bus and attacks if you enter.
I unloaded about 250 rounds of assault rifle, 100 shots of shotgun and two grenades on him. Only effect so far was that a sexond poltergeist appeared...

So fellow Stalkers, is this a bug? If i dont kill him and therefor cant help the scientist, what can i do with those key cards?