It's not about whether the action was actually illegal or not, it's a matter of perception. He perceived there was wrongdoing, brought it up. He did the right thing. You want to judge the action based on your perceived outcome, but that's not how things work. You want people to voice concerns when they have them no?
You will never have a completely impartial complaint. It's not a thing, but you all act like every opinion is immediately invalid because someone has a perceived a bias. Immediately throwing out or silencing all complaints with any hint of bias is going to lead to there being zero complaints. This guy isn't the judge...he isn't determining legality or punishment, he simply raised his hand and said he had concerns and was silenced.
The problem here is the right wing media is painting him as a partisan hack, which then puts the military in a bad light and you don't like. If the media were to simply look at him as a person voicing concerns through an established process (which he is), then it wouldn't be dragging down the "impartiality" reputation of the military. This is a predictable cycle though, as anyone who gives even the slightest shred of disagreement with Trump is immediately eaten alive, even if they are in your own party.
It's easy to get riled up over him when you look at it under these set of political circumstances, but instead imagine a similar situation happening with an IG complaint, sexual assault, a maintainer sabotaging an airplane...etc. You don't want people out there witnessing potentially bad things to be second guessing themselves and being afraid of reporting. Report it and let the authorities sort it out. He did.