Ironically it's a lot easier to come up with motives if the explosion really is an accident and someone survived due to reason X. Which kind of correlates with And then there were none.
If you assume it's deliberate you have to assume there is a motive for a Culprit to destroy everything on the island, and other than the letter about Beatrice "collecting interest" that has yet to be explained. So maybe rather than a motive for murder we should be asking for a motive for a deliberate explosion. Why can't it just be an accident?
EDIT:
And this brings up another sensitive topic. Why does the Rokkenjima accident have to do with Battler's sin? Why does everyone die because he made a promise? We have enough trouble explaining the murders with his sin how does an explosion fit into that?
- Because of your sin, people die.
- Due to your sin, a great many humans on this island die.
- No one escapes, all die.
The only explanations I can come up with are
1. He's cursed by something supernatural
2. the person he sinned against turned (one or more people) suicidal.