While I'm not convinced executing Manning was the answer-- certainly not without trial-- I'd say Life without parole would have been more appropriate.  As you say, the contents of those revealed messages cost  untold deaths and damage to Americans; 
Equally damaging, from my experience, was that the Russians got all that decoded material  which they could match against our original encrypted messages to uncover our various methods of hiding material, thus probably making decoding-- i.e. hacking, -- of our secrets much simpler.
Without being a xenophobic Nationalist I'd say we all need enough patriotism to understand that hurting America hurts us all-- we may not be perfect but there's some really nasty folks out there who'd like to do some real damage here.  It's not a game.

As for Manning-- clearly he/she has some serious emotional problems which provide extenuating circumstances.  I blame the military for this, as a thorough investigation as we had would have uncovered this.