Perhaps it has something to do with System Integrity Protection as well. You could disable it by booting into the recovery OS (hold cmd + r when booting) and then look for the utility to disable it. I wouldn't recommend doing this though. Since El Capitan does bring some new security policies to OS X, it is probably a good idea to wait until the anti-malware software has had time to catch up. Even if you are able to install it now, it might not work correctly anyway. If you really need anti-malware software, use the less intrusive and highly regarded ClamXav.