As per the SIS log
KEY_NAME=[SYSTEM\CurrentControlSet\services\WinDefend]
2013-08-06T18:07:38.159Z INFO I SIS VALUE_NAME=[Start]
2013-08-06T18:07:38.159Z DEBUG I SIS VALUE_TYPE=[DWORD]
2013-08-06T18:07:38.159Z DEBUG I SIS VALUE_DATA=[4]
2013-08-06T18:07:38.159Z ERROR I SIS ZwSetValueKey() failed in thal::RegistryWriteValue() status: 0xC0000022 = {Access Denied} A process has requested access to an object, but has not been granted those access rights.
It failed to stop windows defender by SEP install, hence it rolledback.
However make sure to disable completely Windows Defender in Services.msc also.
Reboot
Symantec Endpoint Protection 12.1.2 Installation failed on computer with Microsoft Security Essentials
if you have security essentials remove that too..