Bitdefender extensions definitely crash Safari in Monterey Beta 5 and Beta 6. I found that the only way to use Safari is to disable ALL extensions using "defaults write com.apple.Safari ExtensionsEnabled 0" in Terminal.
When I uninstall Bitdefender, I can use Safari with extensions, and there is no doubt in my mind that the Bitdefender extensions are NOT compatible with Safari in Beta 5 and Beta 6.
If only there was a way to install Bitdefender WITHOUT the extensions - or remove the extensions in Safari, not just disable them...