not sure anyone who professed to being a Christian would ever become a mass murderer, I would certainly question there salvation any way but either the blood of Jesus covered all sins for all time for every believer or it did not...
What the argument does open if we take it down the route of losing salvation, how many and of what magnitude of sin does it take to negate the blood she for sins??
There is no sin, no amount of sin, which cannot be forgiven through Calvary (save one, but that's another thread)
But to say that if a person who is truly commited to Christ, shows every eveidence of the fruits of the Spirit, and even leads others to Christ, yet, at some point rejects his or her faith is still saved, makes Calvary a whole lot cheaper than it is.
And, yes, I knowe of such a person - now spending time at 'her majesty's' pleasure for a crime - not murder or assault, but a crime for which he is unrepentant.
Is his salvation secure even though he rejects it?