This is all pet theories and silliness for purposes of discussion. I freely admit that I haven't built a case here that's strong enough to withstand even a gentle poking by an opponent.
I don't know as much about arson but I did go through the same serial killer phase as every morose teen and one of the things that stuck with me is the way that some offenders escalate from simple peeping and stalking all the way up to murder. Another thing that stuck with me is how in some cases when there is an intended victim, esp for revenge, an obsessed mind will often hone in on a single characteristic of the intended victim then transfer victimhood to strangers based on that characteristic. The woman who "wronged" you is a skinny blonde who smokes cigarettes so you go out looking for skinny blondes who smoke cigarettes to victimize in her stead because in your unconscious brain that matches the pattern of behavior that would soothe the wounded entitlement of the offender. Given these facts about the nature of obsessive, vengeance-oriented crime and the fact that the serial killer/arsonist crossover is so common that arson is one of the mcdonald triad of behaviors common to serial killers there's a non-zero possibility that we're seeing a revenge fantasy transferred to another victim. There's also the fact that obsessed criminals tend to want to roleplay or practice and a lot of times their first "serious" crime is one of these roleplay/practice sessions getting out of control. This feels like that to me though I can't prove it. It's like he wanted to see what starting a fire would be like, assumed that the local VFD would get it under control and in doing so would also give him an idea of what the response looked like so he could optimize for escape, then either it got out of control or he tried to inject himself into the emergency response (another common thing among obsessed criminals, many like to relive the crime by being part of the investigation, like to tease investigators by being right under their nose or believe that by injecting themselves into the investigation they can steer it away from them).
Again, does any of this hold up in a court of law? Of course not. Does it hold up in a court of a thread on a post on HN? Maybe, we're here to talk and I'm of a mind that we didn't do anything to fix w/e it was that made people serial killers but there aren't really any serial killers anymore so something must have happened to that behavior. Perhaps stranger arson is a way that the same drivers that led to serial murder before the ~~panopticon~~internet are driving new behaviors now. Intuitively I'm highly confident that the stranger spree killings we see now are driven by those same pressures in a lot of perpetrators and the change in MO is about taking advantage of lag time in law enforcement's ability to correlate facts. Before the internet you could drive a few hours' down the road and start using a new name and unless your old name was already in the system there was basically no way for anyone to know. Obsessed criminals could offend, disappear and wait it out. Nowadays we're really good at ID'ing an offender so obsessive murders have to be one and done, but another strategy could be crimes that are small enough that they don't trigger the kind of dragnet response that involves things like checking all the CCTV cameras in a ten mile circle around the crime and things like that.
edit: everyone seems focused on the "serial killer phase" line that was really intended to be a throwaway. I just mean that I read a lot about them and thought it was shocking and cool to have a "favorite". Gross shit, but I assure you no one was ever in any amount of physical or psychic danger beyond declaring me a pizza cutter (all edge and no real point).