frumnotyeshivish
Yes Im aware of that Rav Dessler. He may not have Bechira at this point but I believe the reason he is responsible for what he does now is for allowing his bechira to be taken away by making bad choices up to this point even though technically at this point he couldn’t control himself. Meaning, just like Paraohs bechira was taken away at some point I think after makah 5 so the meforshim ask how can he be punished, and they answer because the first 5 times he still had Bechira and chose not to do the right thing and not to accept what Moshe was saying bsheim Hashem.
I think what Rabbi Shafier means is that even if they were preached to from youth that you must kill “the infidel” internally we all know murder is bad. So if he chooses to kill and ruin his natural good conscience that Hashem gave him then he is responsible for that.