No, that should have been and was enough reason to take it away, but not to keep it. There needs to be a due process. In this case though the process failed at the mental health evaluation, where they authorised his gun to be released.

Even then, we don't know much. For all we know when evaluated (several months after his gun was taken away) he said he didn't feel like a risk anymore and showed all the signs of a geniune recovery, then relapsed after.

Personally, I think the return of the gun should have been a long time later, say a year, after he had clearly shown he was level headed enough in his day to day life. Though I'm sure the NRA has struck enough fear into anyone who would even consider that.