I came across a managed-package on github and wanted to know if there is a way to install this on a developer org as an unmanaged-package. Insights are much appreciated.
It depends on the package. If the package contains hard references to its managed package namespace or any code which can only be deployed into a managed packaging org such as @deprecated, you will be unable to install it into anything but the packaging org.
Another thing that can block deploying as unmanaged code is if the package.xml contains the namespacePrefix, postInstallClass, or uninstallClass elements which can only be deployed to a managed packaging org. However, you can simply remove those elements from package.xml and deploy if that's the only holdup.