uuid_kill is an API command, not a dialplan application. It might be cleaner for you to use the execute_extension dialplan app to execute a simple dialplan extension that uses the set app to execute the API per this wiki entry:

https://confluence.freeswit.ch/display/FREESWITCH/API+Commands

(see "from the dialplan" section)

Another advantage to using the execute_extension app in your bind_meta_app call is that you can have other things occur like spitting out log info.

<action application="bind_meta_app" data="# a b execute_extension::${uuid_of_b_leg} XML foo"/>

<context name="foo">

  <extension name="hangup b leg">

    <condition field="destination_number" expression="^(.*)$">

      <action application="log" data="INFO Hanging up B leg uuid of $1"/>

      <action application="set" data="res=${uuid_kill($1)}"/>

    </condition>

  </extension>

</context>

Please note that the standard disclaimer applies to the information I've presented here.

-MC