I can easily imagine that state hackers in Russia would want to penetrate the DNC. It's full of great political information on one of their rivals.

Releasing it? I'm not sure if that is traditional spy craft.

Julian Assange has hinted that the WikiLeaks dump was originally sourced from an insider.

It's not impossible that BOTH narratives are true.

Conspiracy theories:

I'm not entirely convinced that it was Russia. They point to an IP address that is a proxy server in Russia. If it was Russia, wouldn't they put the proxy server in another jurisdiction to throw people off the trail?