Just because the victim doesn't know/recognize their attacker and wasn't doing anything to "provoke" the attacker at the time of the attack.. doesn't mean there isn't a motive behind the attack.
In my opinion there is more to this story...be interesting to see what unfolds over time.
Just as an example and I'm not saying this is what happened in this situation....but using this as an example to support the comment above;
Victim molested a child 15 years ago....child grows up with all kinds of issues stemming from the incident...happens to cross paths with or seeks out
the molester/victim to exact revenge ..in this case a Molotov cocktail to face!