Somebody asked me that same question the other day :

since yesterday my ubuntu 12.04 locks me out, i can only access the guest account.
if i enter the correct password for the admin account,
the screen turns black and then goes back to the log in screen.
if i enter an incorrect password, it says incorrect password.
if i create a new admin account via the guest account, i get the same problem.
resetting the password won't help :(
any ideas anyone?

It seem's a few people have had this problem, and using gdm instead of lightdm is the recommended workaround, and it worked for the person that asked me - after a couple of reboots. Have a look at these links for instructions & let me know if it helps.

https://askubuntu.com/questions/130387/stuck-at-login-screen

Can't log in to ubuntu 12.04

https://askubuntu.com/questions/130721/12-04-lightdm-cannot-login

Updates

The LionSec Katoolin GitHub webpage clearly warns Katoolin users to be careful when updating software.

Before updating your system, please remove all Kali-linux repositories to avoid any kind of problem.

All kinds of problems can occur if you don't do this.

Software installation

Katoolin is a nice package management tool, however because Katoolin is a package management tool, problems can come in groups of packages instead of one package at a time, especially if you try to grab all the Kali linux tools at the same time. Many of the tools in Katoolin are otherwise only available as .tar.gz archives, and in order to install them, you would have to track them down and compile them one by one. Before you compile a .tar.gz, you should check whether the executable is provided by another package from the default Ubuntu repositories.

In Katoolin Kali linux tools come from multiple repositories and in these repositories there are more than 300 tools. Some of these tools are duplicates of tools that can be installed from the default Ubuntu repositories, and sometimes they are more up-to-date versions having different dependencies than the same tools from the default Ubuntu repositories. Some tools in Katoolin are packaged as standalone tools, whereas in Ubuntu the same tool is bundled into a larger package that contains additional software. When you add a new Kali repository apt doesn't always know if you are adding conflicting packages to your system's list of software, but you'll find out about it when you try to install the packages. Katoolin is a cross platform package management tool, and the same package that installs perfectly on another operating system can cause errors when trying to install it in Ubuntu.

In Katoolin tools can be installed in three different hierarchical levels of grouping.

1. Install all Kali linux tools.
2. Select a category and install all Kali linux tools in the selected category.
3. Select a single tool and install it.

Don't be reckless. In order to install Kali linux tools in a safer way, don't install any tools that you don't need. The more Kali repositories that you add to your software sources and the more Kali linux tools that you install, the higher the probability is of breaking something.