A better approach 

WITH RECURSIVE
    comment_tree (comment_id, level, path) AS (
        SELECT comment_id, 0, array[-score, comment_id]
        FROM comment
        WHERE thread_id = 2
        AND parent_comment_id IS NULL

        UNION

        SELECT
            c.comment_id, level + 1,
            ct.path || -c.score || c.comment_id
        FROM comment c, comment_tree ct
        WHERE c.parent_comment_id = ct.comment_id
    )
SELECT
    c.comment_id, c.parent_comment_id AS parent, c.score,
    lpad('', 4 * ct.level, ' ') || c.text AS text
FROM comment_tree ct
JOIN comment c USING (comment_id)
WHERE c.thread_id = 2
ORDER BY ct.path;