In 2014, Washington State passed Universal Background Checks for all firearms transfers including private transfers.
The type of weapon used is irrelevant, since he didn't shoot 50 people in rapid succession..
So, what we have again is mental illness, however noticeable by friends and family, failed to be reported to the authorities. People say, "If he had been required to attend a mental health exam, they would have seen how depressed he was."
That's called abdicating your responsibility, expecting the system to figure out who is mentally ill through some test. The system is flawed. Do not depend on the system.