While I agree it's useful to look at the bigger picture to avoid future mistakes. He pulled the switch, the buck stops with him. Could be fatigue, could be bad training, but ultimately for the purpose of the investigation it's human error.
If the pilot was at fault then the only unprofessional act would be to not state so. But likewise with any other deficiencies, from material sourcing to engineering process to flight operation.
This. At first, I wondered why the NTSB was getting all the design data from the company, much of which must be proprietary and, as a one-off custom design craft, not useful to the NTSB. I mean, wouldn't the company want to do its own research and find out what happened so it can fix it and resume flights? But then I realized that it's better for everyone if an outside party learns all it can and essentially debugs their design, like Feynman did with the space shuttle. One might even wonder if they aren't getting some highly expert aerospace consulting done for free.
Even if he might have caused it, playing the blame game does not feel very professional. I would be very careful before making such a statement.