Let’s face the facts: we don’t know what was in Hitler’s heart when he died. But we can know, with relative certainty, that the Bible does not teach such a thing. Consider this verse:
Don't be misled; remember that you can't ignore God and get away with it: a man will always reap just the kind of crop he sows! If he sows to please his own wrong desires, he will be planting seeds of evil and he will surely reap a harvest of spiritual decay and death; but if he plants the good things of the Spirit, he will reap the everlasting life that the Holy Spirit gives him. (Gal 6:7-8) TLB
The Thief on the Cross is often used as the example that someone can be saved just prior to death. But a thief who steals out of hunger or need is a far cry from some monster who sent untold thousands to their deaths out of pure hatred.
There’s this idea that God is so forgiving. While that is true, He’s no fool. Scriptures clearly teach that He has and does withhold His love from certain persons. This was true with many Jews who rejected Christ. (Mark 4:12) Blasphemy against the Holy Spirit is an unforgivable sin. (Mark 3:29)
It's not surprising that many think they can live like there's no tomorrow and then slip into heaven at the very end. This attitude is an ancient sin. Look what God had to say about it:
The Lord God called you to repent, to weep and mourn, to shave your heads in sorrow for your sins, and to wear clothes made of sackcloth to show your remorse. But instead, you sing and dance and play, and feast and drink. “Let us eat, drink, and be merry,” you say: “What's the difference, for tomorrow we die.” The Lord Almighty has revealed to me that this sin will never be forgiven you until the day you die. TLB (Isa 22:12-14)
Never forgiven! Ooh, I thought blasphemy of the Holy Spirit was the only unforgivable sin. The only difference is that Jesus said blasphemy against the Spirit would not be forgiven in this age or the one to come. It was to be an everlasting rule.
On top of all this, nowhere does the Bible teach that “asking Jesus into your heart” is the method for salvation. You must believe (John 8:24), confess before men (Rom 10:9), repent (Luke 13:3; Acts 2:38), and be baptized (Acts 2:38; 22:16; Gal 3:27). I don't think Hitler had time to do all those things, much less the inclination.