First option - Repair avast from add/remove.
*Second option - Uninstall avast via add/remove and re-install avast
Third option - Disable Avast Online Security, having to disable it you are still protected with online threats by Web Shield
There’s a tool called avast uninstall utility and it only works for Windows Operating system. Since you are using MacBook Pro I don’t know any tool that is similar to Avast uninstall utility compatible with Mac OS.
Since you are using Mac OSX, please post your Avast issues/problems that you are facing into the Mac OSX forum for Avast here http://forum.avast.com/index.php?board=5.0
This forum is only for Windows Operating system with Avast