It's been a while since you've posted, but I just want to chime in in case it'll be useful for others - in cases like yours, the problem is most likely with Mac's security settings.
Personally I use CleanMyMac and it gives me a safe acces to Sigil, you can read about it on
https://setapp.com/lifestyle/the-best-antivirus-for-mac.
One can also try opening the security settings or preferences and looking for application security-, virus scanning-, or gatekeeper settings options.
On macOS, go to "System Preferences" > "Security & Privacy" > "General" and check the "Allow apps downloaded from" section. Make sure it's set to "App Store and identified developers" or "Anywhere".
In any case, proceed with caution when allowing apps from unidentified developers.