This is a extension that helps you killing any running application.
To install the extension, you need Alfred and its Powerpack. Then, you just need to download and open this file.
Once installed, you can use it in two ways, first by directly writing kill
plus the name of the application, for example:
kill Twitter
kill Finder
kill Google Chrome
Or by selecting first the application in Alfred, then pressing the right arrow and then writing kill
(so that the Kill Application action is selected).