What about obfuscated executables embedded in the js? This is exactly RAA. It downloads other malware, but if it can't download, it still encrypts, executing the embedded executable. My point is, from this video, you cannot know for sure that is was a third payload downloaded from the internet that did the encryption. And if was embedded in the js, that's even worse, it's a static file that should have been detected. If this is the case, maybe the AV installed didn't got an update with the signature for this particular ransomware, or they didn't got the chance to build one (0-day variant).
Anyway, Avast is good, WD is good. I would probably choose Avast, it has to be at least a little bit better, why else would they release another free AV while WD is around?
Windows 10 built in security(UAC,SmartScreen,Windows Firewall & Defender)is much better.
Personally I like Avast & Windows Firewall with UAC to max as well as SmartScreen enabled.
Give Windows 10 built in a try since you have already used Avast but for the poll my vote went to Avast.
In my opinion Windows Defender scan is nothing compared to Antivirus software’s. As Windows Defender is not capable to recognize internet threats while browsing. Whereas the antivirus software with internet security protection is able to detect sites filled with malware and virus. So I recommend you to install a good AV rather than relying on windows defender.