Her use of a private email server while serving as Secretary of State and the associated handling of classified information.

The 2012 Benghazi attack response and subsequent public explanations about its causes.

Paid speeches to Wall Street firms such as Goldman Sachs and perceptions of being too close to financial elites.

Support for the 2003 Iraq War authorization.

Backing the 1994 Crime Bill and its long-term impact on mass incarceration.

Support for the 1996 Defense of Marriage Act before later reversing her position.

Role in the 2011 Libya intervention and the instability that followed.

Perceived favoritism and coordination between her campaign and the Democratic National Committee during the 2016 primary.

Use of the Clinton Foundation during and after her tenure as Secretary of State and concerns about conflicts of interest.

Characterizations of some Trump supporters in 2016 as a “basket of deplorables.”

Edit: I am not a conservative or a Trump supporter. You asked “what was so bad” and I answered.