διὰ τοὺς ἀποκτείνοντας οὓς οὐ δεῖ is “because of people who kill (those) whom (they) shouldn’t.” οὓς οὐ δεῖ serves as the object of ἀποκτείνοντας. δεῖ is impersonal (so it has no expressed subject), and ἀποκτείνειν is understood with it, just as “kill” in understood after “shouldn’t” (οὐ δεῖ) in my translation.
You see how the sentence is constructed: οὔτε … οὔτε “neither … nor” duplicating (but not canceling) the initial negative of οὐ δίκαιον.
— As to your newer query, about copying text from Perseus, you could copy-and-paste into a Word document (or whatever you use for word-processing), and transfer that into your textkit query.
Many thanks mwh for that commentary. After reading it, I saw that I was even more confused than I had imagined! Very helpful. I had misunderstood οὓς οὐ δεῖ, and you set me straight.