Every actions from inside and outside Iran suggest it's more likely an accident. Iran doesn't seem to be blaming anyone and no one are taking any responsibility of the incident. The president himself is actually a bad target even for the enemy of Iran.
The good side of the death of current president is Khamenie now have to chose another person. He's already at his old age and has a lot pressure within the ruling elites as well from the general public. To make the people happy, he might go with a moderate candidate that people would at least have some hope on.