Hi Anne,
In your question you aren't clear about a couple things. We aren't saying a horrible death means one isn't a believer. What about the Lord Jesus Christ, crucified on a cross? And then there are those that believed on Him - Stephen, who was stoned, or James, who was killed by the sword, and the other accounts in the Scriptures. We know these died in, and as a result of, genuine faith, and not just because it is written in the Bible, but because we share in the same gift of faith they received.
Also, one can be a believer at some point, a "Christian" if you will, turn back to the world, and there's no question such a decision is highly likely to include a horrible death, seeing how it's being considered worse than death by stoning:
Hebrews 10:28-29 MKJV
(28) He who despised Moses' Law died without mercy on the word of two or three witnesses.
(29) Of how much worse punishment, do you suppose, will he be thought worthy of punishment, the one who has trampled the Son of God, and who has counted the blood of the covenant with which he was sanctified an unholy thing, and has insulted the Spirit of grace?
What could be worse than being stoned to death? Aside from a variety of other types of death, what about being kept alive, yet in a place of torment, be it in conscience, or in body or soul, and such that you can't escape?
I take your double questions marks ("??") to mean you aren't asking, but gasping at what you are hearing. Can you show us an example in the Scriptures, or elsewhere, that proves otherwise? There are multitudes of so-called Christians who take upon themselves His Name in vain, perish by the thousands every day in all sorts of tragic ways (whether suddenly and prematurely, or over time, even unto old age). Had there been repentance, it wouldn't have happened. Isn't this exactly what the Lord said?
Luke 13:1-5 MKJV
(1) And some were present at the same time reporting to Him of the Galileans whose blood Pilate had mixed with their sacrifices.
(2) And answering, Jesus said to them, Do you suppose that these Galileans were sinners above all the Galileans because they suffered such things?
(3) I tell you, No. But unless you repent, you will all likewise perish.
(4) Or those eighteen on whom the tower in Siloam fell and killed them, do you think that they were sinners above all men who lived in Jerusalem?
(5) I tell you, No. But unless you repent, you will all likewise perish.
Here are some additional Scriptures:
"And he said, If you will carefully listen to the voice of the Lord your God, and will do that which is right in His sight, and will give ear to His commandments, and keep all His Laws, I will put none of these diseases upon you, which I have brought upon the Egyptians; for I am the Lord who heals you" (Exodus 15:26 MKJV).
"As the bird by wandering, as the swallow by flying, so the curse causeless shall not come" (Proverbs 26:2 KJV).
"For the LORD God is a sun and shield: the LORD will give grace and glory: no good thing will He withhold from them that walk uprightly" (Psalms 84:11 KJV).
Job 36:5-14 MKJV
(5) Behold, God is mighty, and does not despise; He is mighty in strength of heart.
(6) He will not keep alive one who is wicked, but gives right to the afflicted.
(7) He withdraws not His eyes from the righteous, but they are like kings on the throne; yes, He causes them to sit forever, and they are very high.
(8) And if they are bound in chains, and are held in cords of affliction,
(9) then He shows them their work, and their sins, that they have behaved themselves proudly.
(10) He also opens their ears to teaching, and commands that they return from iniquity.
(11) If they obey and serve Him, they shall spend their days in blessedness, and their years in pleasures.
(12) But if they do not obey, they shall perish by the sword, and they shall die without knowledge.
(13) But the ungodly in heart heap up wrath; they cry not when He binds them.
(14) Their soul dies in youth, and their life ends among the sodomites.
Also, you can read the following paper:
Do Faithful Christians Die from Terrible Diseases?
Ronnie