Filter posts by tax (dropdown) and meta value

To do the taxonomy query in your case, the code would be like this:

if (isset($_POST['cat']) && !empty($_POST['cat'])){
    $args['tax_query'] = array(
        array(
            'taxonomy' => 'roadkill_category',
            'field' => 'id', // or slug
            'terms' => $_POST['cat'],
            'operator' => 'IN'
        )
    );
}

If you have further questions about tax query, please refer to the Codex.