Why does the hacker fear Hutch after seeing his file?

Once she gets a look at Hutch's file it's obvious to her that he's an incredibly dangerous person.

As Wikipedia explains

Hutch is a former "auditor", an assassin employed by intelligence agencies to kill people who were considered untouchable or too difficult to arrest

He's not someone you want to be on the bad side of so she quits and makes a hasty exit for her own safety.

The key to understanding this is the brief glances we see of the file's content

The hacker originally could not find out much about Hutch and Yulian was a little unhappy she didn't find more data and failed to recognise "a wolf in sheep's clothing". So she exploited her connection at the Pentagon.

We see his attempts to get the data. His first attempt to search the electronic system gets an "access denied" rebuff. But he has access to the paper archives. He successfully retrieves the relevant file but, as we get a brief glance of, most of the content is redacted even in the secure archive. He copies the file to the hacker and, as soon as she sees how much of the detail is redacted, she realises that he must be a very dangerous person indeed. And she wisely departs before his killing spree in Yulian's headquarters.

The point being that any person with a highly secure military file containing so many redactions must be someone who did things absolutely nobody in the military ever wants anyone to know about. Like off-the-books terminations with extreme prejudice of people like the hacker or Yulian.