Anybody commiting crimes, foreign or at home, should be punished equally by the law without discrimination for the job, race, sex etc. The info Manning did released will help to sentence soldiers who did horrible things but wouldn't have came known otherwise. That is a good thing, not to say a very good one, especially because sentencing these soldiers might patch back a bit for what they have done. A bit. Which is already better than nothing.
However, leaking military stuff ain't like giving cookies to poor kids. IT is a VERY serious thing. Manning knew it. He still did it. With style, he did leak... everything. Which means he didn't separate the infos, what people should rightfully know and what is too sensitive and could compromise the life of rightful officials and soldiers on the field. HOw many gigs was that already? That, on the other hand, was a cruel mistake. I didn't skim through all the documents but I am certain that some people are now at risk because of some of the infos.
For this reason, it is very intricated, especially because it is entangle in a world of rules and laws and when one is absolutely found guilty, it's quite clear what's heading. Some greater criminals might deserve to be sentenced and aren't, more than Manning, but opposite to him, they can (unfortunately) play with the legal frame and get away with it. Sadly, the legal world ain't built well enough.

So: Do other criminals deserve harsher punishment than Manning? Of course.

Were Manning intentions good? I think so.

Did he screw up? Big time by a lack of judgement and just uploading it all without discern about the documents.

Should he be forgiven? Despite his initial intentions.. no. Because he did screw up and because it wouldn't send the right message to the others who just like him are manipulating sensitive informations and who could perhaps make an even bigger screw up than him.