If a build with ZAP running is aborted, the ZAP process remains alive. I discovered this when I notced that I had 10 ZAP processes still running!

I think if the plugin was changed to a BuildWrapper it may simplify configuration and allow for proper termination of the process. This way there'd be a checkbox in the Build Environment to indicate that you want a ZAP process to be started. The Admin and Startup configuration could then be moved to this section, and environment variables such as ZAP_HOST, ZAP_PORT, etc. could be exposed for the build steps.

Here's an example tearDown that handles clean up in a plugin I've worked on: https://github.com/mozilla/testdroid-marionette-plugin/blob/master/src/main/java/jenkins/plugins/testdroid/DeviceSessionWrapper.java#L226