Meaning of まじくは


I've seen this sentence

許すまじくは許さない。俺自身は主義としたそうした一線を持たないが、だからといってそれを持つ人間を軽んじることはしないだろう。

and I would like to know what is the meaning of まじくは so that I can understand
the expression 許すまじくは許さない.

Making an attempt to get some hint through machine translation, I've got the
following.

"Forgive me, but I won't forgive you. I myself don't have that kind of
principled line, but that doesn't mean I don't look down on people who do."

Although I know there may be some flaws in this translation, is 許すまじくは許さない
referring about a moral principal, about a way of being? I must say I'm
somewhat lost when it comes to fully understanding the whole idea of the
original sentence and translating it.




It would seem that this usage is entirely analagous to 詳しくは, 遅くまで, 近くに,
(朝)早くから and so on, where the 連用形 of an adjective is used as a noun, 許すまじくは
thus meaning 'what isn't to be/shouldn't be forgiven'.

In classical Japanese the 連用形 of both verbs and adjectives can be followed by
は, which has the force of ば, forming a conditional, which would also seem to
be a possible interpretation here, 'if it shouldn't be forgiven, then...', but
you wouldn't expect to see this in modern Japanese (I thnk).

