Add this to /etc/apt/sources.list:
deb http://http.kali.org/kali kali-rolling main non-free contrib
Create this file (with nano or vim): /etc/apt/apt.conf.d/81ssl-exceptions
Add this line: Acquire::https::Verify-Peer "false";
then `apt update` and `apt install bettercap`
You'll still get an error for firmware-b43legacy-installater, but bettercap will get installed.