But he didn't get a fair trial and if you read about the case, these points are important: the handwriting evidence that he wrote the original kidnapping note. H said he was told to write the note with the wrong spelling which he did. His explanation of how he had the ransom money in his house - he said Fisch left it but the prosecution never proved that didn't happen. The timber found in his house I believe was used in the ladder and he was a carpenter but it was never proved that H made the ladder or that he took it to Hopewell or that he climbed the ladder and took the child.
It's all circumstantial and while it's strongly circumstantial it wasn't proved beyond a reasonable doubt. The hysteria around the case because it was Lindbergh's child made it almost certain that whoever was put in as defendant would be found guilty.
I also think that if you look at the time of the trial, you have growing unease with Germany because of Hitler. Here's a guy who's come to the USA illegally not once but twice. He's obviously involved. Lindbergh recognises his voice (and that was a travesty of justice allowing that testimony but understandable) so Hauptmann must be guilty.
Think of this case in a court today. With DNA and all our sophisticated CSI, he might be found guilty of being an accessory but that's it. Let's face it, if CSI couldn't convict OJ Simpson, then it couldn't convict Hauptmann.
CSI could have convicted OJ, but I think they got star struck or something because they didn't do their job right at all.