The "mistake," objectively speaking (if that's possible at all in war), was that any net benefit from this killing was offset by the negative press and perceptions of killing a non-combatant. It's not as black and white as some would make it, for example, the assassinations of the appointed leaders in the Russian-occupied territories seems to be much more socially acceptable, and yet they're not soldiers. The difference in this war is that there's been such a gap between Ukraine and Russia with regards to moral standing that an episode like this that potentially muddies the waters doesn't help the Ukraine side. All that said, I think the American response has been appropriate, and we haven't seen (publicly) that similar events have occured in the aftermath. The earlier this is finished the better, and the complete restoration of Ukraine, including Crimea, is the start. The downfall of the Putin regime is the logical next step, and likely should occur internally. Finally, and some will argue on this, but large scale rebuilding efforts for BOTH Ukraine and a post-Putin Russia will be necessary for an enduring peace. My prediction is that China will swoop in on the Russian side to prop up a pro-China government, while the US and EU will rebuild Ukraine.