What is the biblical basis for the forgiveness of sins for very evil persons? LDC asked this question before; it was down voted 8 times before it was closed, probably because of a provocative title and lack of a specific question. My editing of his question is: what are the biblical bases for forgiveness of sins and heaven available to evil persons such as Hitler, serial killers and ISIS terrorists? Is it as simple as just asking for forgiveness only? Thank you for you patient answer.
Yes, the bible says that some sins are not forgiven. There are three sins that I know of: 

the Blasphemy of the Holy Spirit (Matthew 12:31), which many scholars simply consider to be a lack of saving faith in Jesus;

refusing to forgive, or not valuing the forgiveness offered to you freely (Matthew 6:15); and,

"backsliding" (Hebrews 6:6), which is again, apparently, despising the forgiveness offered by Jesus on the cross.

I remember a story about a famous prison minister (I think it was Chuck  ?    of the Watergate scandal) who said that he was suprised by the reaction of Christians when he told them that an infamous serial killer (I think it was the Son of Sam) had confessed and become a Christian before his execution.  But Christians should remember that we have all sinned, and Jesus' Easter sacrifice is cheapened if it only covers some of the our sins.  I know that there is the possibility for murderers (e.g. one of the thieves on the cross next to Jesus) and even mass murderers can be saved, but as with any sin, it is very difficult to become sincere about your sorrow (see post above) if you have expected to use God's forgiveness as an escape clause just before you die.  One possible test of sincerity of the deathbed confession is this; if you somehow don't die (e.g. the Governor pardons you), will you change your life?