MichelV,
Unfortunately it'd appear you've run up against an unfortunate known issue that was specific to 8.1.1 and Windows 2012 R2 / Windows 8.1. The issue is referenced here and detailed here.
During install our Endpoint is configured to run as a Protected Process Lite (PPL) through the Service Control Manager (SCM). Unfortunately, for the 8.1.1 release, the signing issue noted in the links above prevent our service process from successfully starting as a PPL process. Unfortunately once a service is configured to run as PPL with the SCM, only a process running at or above PPL would be able to delete the service to clean up a failed install. And as our signing issue prevented us from running as PPL, we're unable to delete the service from the SCM.
Fortunately, as you noted, a reboot should clean up the issue (we delete the service keys) as discovered. This could be verified before reboot using the reg command:
Unfortunately, further attempts to install 8.1.1 on the host will fail in the same way, but 8.1.2 has a fix and should not encounter the same issue.
Sorry for the trouble.
-Ben