I think it was Seth Rich with a flash drive.
He was mad that Hillary stole the nomination from Sanders, and wanted to make sure she did not get elected.
I don’t think he had anything to do with IT infrastructure support. He was in charge of get out the vote and would have only needed access to that program. I think it unlikely he would have had admin access to the Exchange Server. The DNC IT operaterion was big enough that it had 2 IT staff. They have in house staff and a 3rd party support firm. Not sure who does what, but it is more likely somebody in IT dropped the ball on patching systems or was spearfished to get admin passwords.