Yetzer Harah “Defense” Under Halacha

I am not sure how one could possibly prove that his yetzer horah is so strong that it was impossible for him to do otherwise.

Although when it comes to addictions maybe it’s different?

A better question should have been, Is Not guilty by reason of Insanity a valid defense under halacha. There really are insane people (I didnt say this guy, but in general there are)

“Is there such a mitigating factor under halacha that legitimizes the notion that some individuals are powerless to control their actions against an all-powerful YH?”

Halacha? No. (though we do find by anusa techila where at the end she was not anusa – the Gemara כתובות נא gives a reason יצר אלבשה” – but there has to be Anusa Techila.)
But there is such a concept Al Pi Kabalah/Chasidus; see sefer May HoShaloach of the Ishbitzer Rebbe R MY Leiner ZTL how he explains the story of Zimri in Parshas Pinchas (or Balak), and is quoted by his Talmid Rav Tzodek haCohen.

For the “my hand did it” you answer “so let’s cut off the hand and put it on trial”. Here he’s claiming the yetzer hora took control of his body. Unfortunately for him, if he unable to prevent the yetzer hora from doing so in future since society has a right to protect itself…

There are certain situations which fall below the Bechira level, which – according to R’ Dessler – renders the perpetrator unaccountable for his transgressions. Nevertheless, putting oneself in a situation where one will fall below his level of control is enough to hold him accountable for transgressing.

Regardless of this, however, no-one has ever got himself off the hook in Beis Din for claiming that the sin he did was below his level of Bechira. בית דין של מעלה may accept that, but in our Beis Din, if you do the forbidden act after accepting the warning from two valid witnesses, we punish you.

There is one place where we find the Yetzer Hara defence in practical Halacha, although the parameters of this defence are subject to large scale debate in the achronim. The Gemara in Kesuvos 51b mentions a concept of יצרה אלבשה, which is enough that we consider – even in this world – certain situations as אונס, not רצון.

See also Kiddushin 40a, although not entirely relevant.

(Of course, this does not apply to disgusting people who have taivos and have no self control.)