https://en.wikipedia.org/wiki/July_12,_2007,_Baghdad_airstri...
You're free to argue that the laws of war should be stricter. But even if they were changed, it wouldn't retroactively make this incident illegal. It may or may not have been legal (i.e. it's legally questionable), whether you like or agree with the actions is a separate issue.
Contrast this incident with the case of that Navy SEAL murdering a POW. That's not legally questionable, that's very clearly illegal.
Watching the video is enough to dispel that.
The most legally questionable part (I believe) is firing on the van that was evacuating wounded people. The best ethical defense to that attack was that they didn't know children were in there and the driver is primarily to blame for what happened to them.
But even if it was most likely an ethically wrong decision, that doesn't make it automatically illegal.
And all these arguments are spurious IMO. Collateral Murder exposed what was being done secretly overseas in US citizens' name.
If it wasn't illegal then there should be no problem showing their people exactly what their war budget is paying for.