A Christian dying of an overdose does not send him to hell. He was saved by faith in Christ not his works. All of us Christians sin. You sin, and I sin. If a drug user was never saved that is up to God.
You are correct that the conditions of salvation are determined and set by God.
"Not everyone who says to Me, 'Lord, Lord,' will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter. "Many will say to Me on that day, 'Lord, Lord, did we not prophesy in Your name, and in Your name cast out demons, and in Your name perform many miracles?' "And then I will declare to them, 'I never knew you; depart from me, you who practice lawlessness'" (Matthew 7:21-23).
Notice that Jesus' statement is quite different from your own. Sin does keep people out of heaven. Becoming a Christian does not give you immunity from this fact.
"This is the message we have heard from Him and announce to you, that God is Light, and in Him there is no darkness at all. If we say that we have fellowship with Him and yet walk in the darkness, we lie and do not practice the truth" (I John 1:5-6).
Your response shows that you agree that drug use is a sin, and I agree since it ruins a person's soberness. "For those who sleep do their sleeping at night, and those who get drunk get drunk at night. But since we are of the day, let us be sober, having put on the breastplate of faith and love, and as a helmet, the hope of salvation" (I Thessalonians 5:7-8). Thus, the question is whether a person can be involved in sin and still expect to reach heaven. Paul says "no." "Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality, idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God" (Galatians 5:19-21).
No one has to remain in sin (I John 1:8-2:1), but those who do can only expect to face the wrath of God. "But because of your stubbornness and unrepentant heart you are storing up wrath for yourself in the day of wrath and revelation of the righteous judgment of God, who will render to each person according to his deeds: to those who by perseverance in doing good seek for glory and honor and immortality, eternal life; but to those who are selfishly ambitious and do not obey the truth, but obey unrighteousness, wrath and indignation" (Romans 2:5-8).