Create Logout Link WordPress Admin Menu

You can use the awesome WP_Admin_Bar & hook in with admin_bar_menu:

/**
 * Add a "Log out" link directly to the admin bar.
 * 
 * @link    http://wordpress.stackexchange.com/q/141446/1685
 * 
 * @param   WP_Admin_Bar    $wp_admin_bar
 */
function wpse_141446_admin_bar_logout( $wp_admin_bar ) {
    if ( is_user_logged_in() ) {
        $wp_admin_bar->add_menu(
            array(
                'id'     => 'my-log-out',
                'parent' => 'top-secondary',
                'title'  => __( 'Log out' ),
                'href'   => wp_logout_url(),
            )
        );
    }
}

add_action( 'admin_bar_menu', 'wpse_141446_admin_bar_logout', 100 );