ive considered terrorism not political in nature but more of a type of target: indiscriminate killings and injuries of mass amounts of civilians. it's often used for political ends but the purpose is to instill fear in the people other than those directly effected (which is why it's often politically motivated as it has a lot of repercussions outside of the direct effect of killing people). but if it's an attack on the us government, that's an attack on the us government; it's an act of war or an act of insurrection. terrorism to me needs to go after civilians indiscriminately.
also, religious motivation is a political one. that seems to be the underpinnings of this (religious and other political reasons). although im pretty sure this 19 year old is a f**k moron