I have an iMac that is infected and currently disconnected from the internet, but I need to download a version of bitdefender to my windows laptop so I can put it on a usb drive and run it on my iMac while it is offline.
In short, you will have to use the internet to install bitdefender on your macOS since there is no standalone installer available for macOS. Also the bitdefender windows files will not get copied onto the usb even if you try to do it in safe mode. Though some files will be copied but some system based bitdefender files will not get copied which will lead to non functional programming of the product.
Earlier bitdefender used to have bitdefender rescue disk which used to create a bootable rescue usb or cd so that user could boot directly from the usb or cd and disinfecte their PC. But now bitdefender has stopped developing it. Instead they have integrated it into windows product of bitdefender and when you click on the rescue environment and reboot your PC, the rescue environment opens automatically and helps clear out malicious items, but in the windows OS only.
Now, if you are curious to try and use bitdefender to scan your mac through usb, there is only one available option. Kindly download emsisoft emergency kit from emsisoft website and extract the setup file on your usb. Emsisoft uses malware signatures of bitdefender with their own inhouse engine. Since emsisoft emergency kit is a portable windows utility, it can easily be transferred to usb drive. Rest you will have to figure as how to run windows executable on macOS.
(bitdefender beta tester 2019/ 2020)