Let's try to brown townyze why he left the vault. He obviously is alone, being the lone survivor. I don't think that some other life force killed the other dwellers, given the vault was still sealed shut when he came to open it. I hope that it wasn't something lame like every other cryo tube failed except his survived for no apparent reason, because that'd be kinda lazy, and sort of a reiteration of FO3's storyline. I can't really say why he'd be the only one left alive. Now, as for why he left the vault. It could be that he either saw no point staying there any longer. It's also possible that he's either running away from his problems, and that that it was somehow his fault that everyone else died, or that he's looking for answers as to why they died. He seems like he's kind of on a search for something personal when he returns to his pre-war house and finds Codsworth. He also acts like he needs healing since he just randomly allows the dog to travel with him. Idk, just speculations because there are many ways Bethesda could take this.
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