They Are Not Above the Law
Don't worry, FBI agents are not above the law. Oftentimes, they're investigated by their own bureau. They are supposed to be upheld to the standards of the FBI. If they're found to be breaking policy or using resources for their own gain, they will be investigated and maybe even fired.
Counterterrorism Efforts
The FBI is the top agency in the U.S. to investigate terrorist claims. They cover all of the bases, including domestic and international threats. Their biggest priority is national security.