( The previous answer helped me, Thanks! )
ここで追いつめられて斬ってしまえる程度なら、いっそ笑って見ていてやれるものを。
( It seems ... this [ 程度 ] is not the right word here. This anachronistic construction is a little too advanced for the author and editor, and many readers. ) ( *** See below )
If you (the black haired hero) were the type who could slay the opponent (the nervous, novice kid with the white hair), then we (the 2 malicious Senpai's) would [rather] have just sat back and relished the spectacle.
[next page]
But it's your half-assed principle and code (of conduct) that'll do you in.
いっそ -- ( one meaning is "rather" )
https://kotobank.jp/word/いっそ-434335
デジタル大辞泉 - いっその用語解説 - [副]《「いっそう(一層)」の音変化という》1. 中途半端な状態を排して思いきったことを選ぶときに用いる。とやかく言わないで。むしろ。
いっそのこと。「そんな絵なら いっそ掛けないほうがましだ」 ...........
ejje.weblio.jp/content/いっそ -- ( 都会暮らしがいやになってきた. いっそ郷里の村に帰ってみようか == I've got sick of city life. Shall we (take the plunge and) go back. )
This is the most common use. It's sightly different in the manga page.
.... ものを
https://kotobank.jp/word/ものを-646481
デジタル大辞泉 - ものを の用語解説 - [接助]《「ものを」から》活用語の連体形に付く。1. 愚痴・恨み・不平・不満・反駁(はんばく) などの気持ちを込めて、逆接の確定条件を表す。...
ejje.weblio.jp/content/ものを -- ( だまっていれば いいものを. == He should have kept silent. )
だまっていれば いいものを. == If only you had kept silent. ("If only you had kept your mouth shut.")
( It seems ... this [ 程度 ] is not the right word here. This anachronistic construction is a little too advanced for the author and editor, and many readers. ) ( *** See below )
Ok, i have a theory. i think the more fitting word タマ was first used, and the editor didn't think it appropriate for a PG-13 mangazine, because タマ is a gangster lingo, and has other adult overtones. The editor couldn't think of a good replacement and got stuck with [ 程度 ] .
不良・ヤクザ用語. 『タマ』
「あいつはそんなタマじゃない」と言う時の「タマ」とは何ですか? - あざけり ...
detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1339904918
奴』(やつ)と同じ意味です。ttp://dic.yahoo.co.jp/dsearch?enc=UTF-8&p=%E3%82
%BF... ... 言葉の使い方 よく使われる「あの人はそんなタマじゃない」という言葉ですが