They were struck dead because they lied to God.
Acts 5:1 ¶But a certain man named Ananias, with Sapphira his wife, sold a possession,
2 And kept back part of the price, his wife also being privy to it, and brought a certain part, and laid it at the apostles' feet.
3 But Peter said, Ananias, why hath Satan filled thine heart to lie to the Holy Ghost, and to keep back part of the price of the land?
4 Whiles it remained, was it not thine own? and after it was sold, was it not in thine own power? why hast thou conceived this thing in thine heart? thou hast not lied unto men, but unto God.
5 And Ananias hearing these words fell down, and gave up the ghost: and great fear came on all them that heard these things.
All your quote form Hebrews 9 does is to state that the sacrifice of the Cross was necessary for salvation. It does nothing to advance your apparent argument, that the teaching of Christ given His disciples prior to His death and resurrection are of no interest to us. In fact, thank you, this is the first time I encounter this particular Protestant exegesis, that simply ignores 90% of the Gospel. It is so good that added it to my collection, which you can view on my profile (scroll to the end).
One can argue what the cardinal sin of Ananias and Sapphira was. The fact that their lie was about withholding a part of the proceeds cannot be irrelevant. Surely if they secretly did some paid work over the weekend and added the wages to the proceeds of the sale, then such a lie would not be punished. But be it as it may, whatever their particular sin was, the episode shows that a committed Christian can subsequently lose his salvation.