Why did Guru come back to life?
This is all explained by King Kai during the Freeza Saga in chapter 321.



King Kai asks Kami whether people that had died from old age could be revived, to which the answer was negative. So then he asks whether people that had indirectly died by an evil person could be revived, to which the answer was positive.

Kami replied positive, saying that even though it'd be a first-time case, he'd believe it would be possible to revive those that indirectly died. They would then remain alive for the duration of time their life was shortened by.


  King Kai: What if they were confronted by an evil person, and, indirectly though it may be, their death was hastened, even a little bit...?
  Kami: I believe they would remain alive for the duration of time their life was shortened by...


Knowing this, King Kai asked not only to bring back those killed by Freeza himself, but also by his henchman (Eg: which would include Vegeta).