Take this email, for example:
If this were two Hillary campaign staffers discussing it, it would be very sleazy, but not blatantly wrong.
The problem is that it's two DNC staffers. Since DNC effectively organizes the primaries on federal level, they're supposed to be neutral. Instead, we see people not only expressing a clearly non-neutral opinion on one of the candidates, but they are actually plotting to do something that would benefit one candidate by hurting another.
Contrasted with the official DNC claim that they were, indeed, neutral, this is pretty damning. Not illegal, most likely, but as far as reputation goes, it's going to hurt. And Clinton will be affected by it as well, simply because she was the beneficiary of it.