Getting error while trying to use custom comment function

Question

You should not have the () on the callback field.
You shouldn’t need the add_action so remove

add_action('load_comments', 'custom_comments');

And try:

 wp_list_comments('type=comment&callback=custom_comments');

Also, you should extract the $args so they are available in your function:

function custom_comments($comment, $args, $depth) {
    $GLOBALS['comment'] = $comment;
    extract($args, EXTR_SKIP);
    //etc