Prosecution by the executive of successors (or other branches of government) raises an existential threat to the republic. It's far more important to ensure succession cannot be questioned than it is to punish every felony.
Seriously, while I entertain the possibility of voting for Hillary, I still think the fact she's running for President shouldn't exempt her from the law.
Of course, that brings up the question - was she really breaking the law? Were others before her doing the same?
The problem is that the executive's power to prosecute can be abused to undermine democracy when it's targeted Presidential candidates. Politically motivated prosecution is not unheard of. That's not the risk in this specific case, but that's not a road I want to start walking down.
Of course, that brings up the question - was she really breaking the law? Were others before her doing the same?