If he was sincere, and believed that God was, and He is a rewarder of those that seek Him.... sure. All sins are forgiven but one....continual rejection of Christ's payment for all the other sins (other wise known as blaspheming the Holy Spirit).
1. There is the sinful state.....we all are there. Forgiven already
2. There are the current sins (forgiven already) but exercised by in the current life (you reap what you sow)
3. and then there is continual disbelief (unto death of the body) of the conviction of sin and subsequent salvation for that sin (blaspheming the spirit who is telling you this)......not forgiven. ever.
The above is the Reverse Gospel. It's the scary, intimidating version of the Good Gospel, which goes like this.....Jesus is the life. the truth and the way. No man comes unto the Father but through Him.
So if adolf hitler was true, then God chose him to be so and would have granted him life. Consider the two theives on the cross with christ. One believed Jesus was sent of God and was going back to him when he died. The other didn't. A picture of the whole existance of man and God. And all at about 11:59 pm. It's not when, it's if you believe God that seals your fate forever. Even hitler's.