Let's refocus on the basics.
Death in Scripture doesn't mean a state of nonexistence, nor does it only refer to the body separated from the soul.
Death is a state of existence involving separation. In man that might be a separation of body from soul, soul from Spirit, body from spirit.
Faith without works is indeed dead. This doesn't mean faith without works is a separation of the soul and spirit or the soul or spirit from God. Faith without works merely discerns an action of the soul from an action of the body and possibly in some contexts, an action of the soul separate from the activity of the spirit.
Recall Christ commenting about those at a family funeral, to leave them and allow the dead bury the dead.
In the example you provide or Mother Theresa type discerned with an Adolph Hitler type and posit the Hutler type exercising a little more faith than absolutely no faith, still constituting a saving faith. Then you provide an exclamation if one really believes both could go to heaven.
This misses the entire perspective of salvation. Heaven is not a reward for being good or not being bad. We all have been born condemned to everlasting torment prior to any human's salvation (except for Adam and Christ). With respect to disobedience to God, i.e. sin, we are all condemned prior to salvation. This included Mother Theresa as well as Hitler. When we exercise faith in Him, God the Father is free in His perfect holiness (comprised of His pefect righteousness and His perfect justice) to bestow by His grace, a regenerated spirit in us. He also doesn't look at us when we approach Him, rather He looks upon our Lord and Savior Christ Jesus, whom was judged for the sins of the world, past, present and future, at one time having them all propitiated and judged by the Father. From Romans 3:22 we see that the faith of our Lord and Savior Christ Jesus is unto all and upon all that believe in Him. Accordingly, if even Adolph Hitler in his early childhood or in his bunker sometime before he committed suicide, decided to exercise just a little bit of faith more than absolutely no faith, then he still would have exercised a saving faith that would have set up a situation where God may have bestowed His grace upon him with a regenerated spirit, hence an eternal life, predestined for heaven.
We do not know if this occurred, that is between God and the believer.
If one continues to sin, or procrastenate a saving faith, he isn't getting away with anything. Instead, there are rewards predestined for us in heaven with our Lord and Savior Christ Jesus as somewhat of an escrow officer, awaiting to reward us with crowns in heaven. Those who fail to abide in Him, by faith, merely remove themselves from being in the right place at the right time to perform works, perhaps, mental, perhaps physical, that may be judged as divinely righteous and worthy the predestined reward. When we fail to remain faithful, we merely 'leave rewards on the table' so to speak, somewhat like an eternal memorial to our stupidity and consequence of good and evil independent of God.
There are many obvious sinners devoted to evil or lasciviousness. That is a form of immoral degeneracy for Christian who fall away or lapse into reversionism, some also called carnal Christians. There are also a very large number of moral degenerate Christians, who have also fallen out of fellowship with God, but instead focus on self and legalism or possibly asceticism. Their activity is frequently confused and labeled as 'good' but hardly qualifies for divine good or divine righteousness, wherever that person has stepped out of line with God's plan and hasn't returned to God on His terms, through faith in Christ.
Do-Gooders, legalists, moralists, Crusaders, all fall into this category of persons who perform works that will be burnt up as good for nothingness.
Quite truthfully, if somebody like Mother Theresa, happened to have sinned slightly, then instead of confessing her sin and repenting or turning away from an independent thinking, and failed to return to God through faith in Christ ONLY, on God's terms, then that person might very well attempt to overcompensate by becoming very, very moral, but nevertheless, good for nothingness in any conseuent soulish behavior.