A small tip about deleting the Content Type. Usually all of us, will be trying to delete the Content type, before remove the association from the List and Libraries. At that time, we will get an error message stating that,
Another site or list is still using this content type. If you would still like to delete it, please remove the content type from all sites and lists and then try again.
Then, we will go back to the SiteCollection, remove the association, and delete the List/Library itself.
But even then, we will not be allowed to delete the Content Type. A simple mistake, at least every one of us would have done once. A simple stuff, we need to remember is, delete the List/Library from the Recycle Bin as well. Then, come back and delete the Content Type. It will allow us to delete the content type.