De Wikipedia :
The main purpose of a killbit is to close security holes. If a vendor discovers that there is a security hole in a specific version of an ActiveX control, they can request that Microsoft put out a "killbit" for it. Killbit updates are typically deployed to Microsoft Windows operating systems via Windows Update.
A killbit is a flag in the Windows Registry that marks a CLSID as being unsafe. The CLSID acts as a serial number for the software in question — a unique GUID that must exist for each piece of software that behaves as an ActiveX control. If an ActiveX container finds that the CLSID of a killbit entry matches the CLSID of the software, then the software is blocked from running in the ActiveX container. If a vendor wants to release an updated version then they release it with a different CLSID.
Então, basicamente, a lista KillBits é uma lista de componentes ActiveX que nunca poderão ser executados em seu sistema devido a falhas de segurança. Esta lista precisa de atualizações regulares.
Você também pode conferir aqui algumas informações mais elaboradas sobre KillBits do que as ofertas da Wikipédia: link