Are YOU a pilot? At FL350 you wouldn't be instantly blue in the face and pass out... Do you KNOW how long it takes to don a quick-donning oxygen mask? It takes me two (2) seconds to pull the mask out of the slot with one swoop of my left hand and plant it on my face.
That's not my point, there was a chain of events that led to the Helios crew not donning their masks, and all I'm saying is that with the limited information at hand, depressurization is the most plausible scenario as it explains strange crew behaviour and strange flight path.
It may have been a hi-jacking or suicide but very unlikely IMO.