『ハリー・ポッターと賢者の石』本
I don't suppose it could hurt to tell you that.
それくらいなら言ってもかまわんじゃろう
*hurt：[否定文で] 《口語》〈人に〉差し障る・不都合である
Another glass won't hurt you. もう 1 杯ぐらい飲んでも差し障りはないでしょう.
(研究社 新英和中辞典)
*it wouldn't hurt to (have or do something)
It is or may be good, pragmatic, or beneficial to have or do something. A name or personal pronoun can be used between "hurt" and "to." (Also used with "won't hurt," "couldn't hurt," "doesn't hurt," and "can't hurt.")
(thefreedictionary.com)
supposeもthinkと同様否定を主節でする動詞。
復習：
"I think + 否定文"ではなく"I don't think + 肯定文"
ロイヤル英文法Ｐ757では、I don't think と否定して、肯定文を続ける仲間を挙げています。
「～と思う」
believe ・ expect ・ fancy ･ guess ・ imagine ・ suppose ・ suspect
「～のようだ」
appear ・ seem
否定語が後にくることが多い動詞も挙げています。
be afraid・fear ・ hope ・ wish ・ assume ・ trust ・ presume