If you have issues with WideMail crashing please try the bug workarounds in my previous post and also check what other plugins you have installed in Mail.app WideMail has a few incompatibilities with plugins such as: GrowlMail, Letterbox.
To completely remove WideMail from your system follow these steps:
Delete the files located at:
Where ~ is your home directory
~/Library/Mail/Bundles/WideMail.mailbundle
~/Library/Preferences/com.daneharnett.widemail.plist