Various users already found Nagstamon being detected as malware – see https://github.com/HenriWahl/Nagstamon/issues/528. Despite different virus scanners scanning results (https://www.virustotal.com/#/file/d89d45c7442cf006629607b0ab21301f6d688faedf12041b42b1a2beadbdafb3/detection) Nagstamon is not virus infested.
The binaries for Windows and macOS are packed with PyInstaller from https://www.pyinstaller.org. According to their issue tracker they already know the problem of being detected as malware, as noticed in https://github.com/pyinstaller/pyinstaller/issues/680. They also state that there is not much more to do than telling the antivirus software vendors that they are wrong.
So there is no real solution we could provide – sorry to say that, but just tell your antivirus software makers to fix their detection engines.