Did Jesus die for the sins of Hitler? Well, John the Baptist said, of Jesus, "Behold the Lmab of God, which taketh away the sin of the world."
The sin of the world seems like a pretty all-encompassing thing.
Paul, when speaking of the gospel, said, "Christ died for our sins according to the scriptures" If this is the gospel, which is preached to the LOST, then the "OUR sins" would be speaking universally.
Jesus Christ died for ALL the sin of ALL the world-He paid for the sin of the entire world, all the lost. Thus, yes, even for Hitler and Jeffery Dahmer and Ted Bundy and Joseph Stalin and Osama Bin Laden, etc, etc.
The question is, Is this to be considered "universal redemption"? No, redemption is to possess something and then, one day, redeem it to yourself, like buying something on layaway-you can pay for it, but leave it until a convenient time to pick up. It is bought and paid for, your property, but to be redeemed at a later time. So, Christ paid for the sin of ALL, but not all will be redeemed, because unlike layaway, the product paid for has to be in agreement-they have to accept that payment for themselves. So, some reject that Jesus paid for them, some disbelieve it, some still want to try and pay for themselves.
Salvation is like having a gift for someone-its bought and paid for, and I am holding it in my hand, offering it to them-but that person must reach out and accept the gift, or it does them no good-it is theirs, it is paid for, but not accepted. We MUST accept salvation for the salvation to apply to us.