This attack on a GOP campaign office in North Carolina is a very serious situation. Any kind of election related violence is always serious. But it is especially so in a campaign which has already seen more incitement and incendiary than in almost half a century and arguably much longer. Predictably, Donald Trump has publicly blamed Hillary Clinton and cited the attack as a reason that "now we have to win." In other words, Trump is now arguing that victory is either necessary as payback for the fire or that victory is necessary to defend supporters against future attacks.
The ATF is involved in the investigation. I think it is wise not to make too many assumptions about the intentions or identity of the arsonist. On its face, the attack looks like it is anti-Republican in nature. But recent elections have also witnessed a number of incidents, either attempted or otherwise in which supporters of one party carried out attacks either on themselves or their own partisans in an effort to tarnish the other party. In other words, false flag attacks, usually of an extremely clumsy and quickly discovered nature. Lots of places have surveillance cameras these days. I don't say this is likely, only that it is a real possibility based on recent history. With federal law enforcement involved there's a good chance we'll know more soon.