The shutdown.sh script try to bring the server controlled out of work, e.g. remove from cluster, finish all transactions and so on.
The script sends only a request to JBoss core that the server should be stopped (can also be done by kill -1).
The script does not work in different circumstances (e.g. massive GC).
If you want to stop the JVM immediately you might use 'kill -9' but this may affect db connections.