Why is this program considered a bad application?
This is probably a false positive meaning that the antivirus solution threats the package as infected although it's not. I've downloaded the software from its official website and installed it while having BitDefender installer and everything was normal. The antivirus wasn't triggered and FastStone Image Viewer installed correctly.
Redownload the software from its official website and install it, then add it to the exception list in case the antivirus detects it as bad.
I have used for many, many years, downloading updates from the Authors site, and never gotten any type of virus warning scanning with both Kaspersky and Norton scanners. It does not send any data, through any network port, that I have ever detected