He was described as "a recluse", which seems to indicate he wasn't part of any organised plot.

He was the sort of foot soldier certain organizations like to inspire so they can use and disavow them. White supremacism is certainly organized in the US.

There was no organized logistical backup for most of the classic 19th century terrorists either. Gavrilo Princip, with a serious political movement helping him out, was not a typical assassin.

A "terrorist" seems to describe a fanatic ready to kill and die for a cause - don't think there's too much evidence of that to date.

His manifesto is completely explicit about his cause.