Helios was caused by the pilots misundertanding the cabin altitude alert for a faulty take/off configuration alert (same alarm). At no point did they suspect a failiure to pressurise and they never donned their masks, unlike the passengers whose masks dropped down in front of them.
No doubt had the masks dropped down for the pilots they would have got the message, but instead they messed around calling maintenance slowly becoming hypoxic until it was too late.