Question

Makkot 16a:
We learned there: If one takes the mother bird along with the young, Rabbi Yehudah says: He is flogged and does not send her away; and the Sages say: He sends her away and is not flogged. This is the general rule: Any prohibition that contains a positive corrective commandment, one is not liable for it. Rabbi Yohanan said: We have only this one and one other. Rabbi אלעזר said to him: Where? He said to him: When you find it. He went out, analyzed it, and found that it was taught: In the case of a rapist who divorced his wife—if he is an Israelite, he takes her back and is not flogged; if he is a priest, he is flogged and does not take her back. This works well according to the one who teaches “he fulfilled it / did not fulfill it,” but according to the one who teaches “he nullified it / did not nullify it,” granted, with regard to sending away the mother bird you can find such a case, etc.
And see Rashi, s.v. “Rabbi Yohanan said: We have only…” — “That is, he sends her away and is not flogged; but if he did not send her away, he is flogged. And Rabbi Yohanan holds by ‘he nullified it / did not nullify it,’ and he understands the phrase ‘he sends her away’ to mean: he sends her away whenever he wants and is not flogged. And when is he flogged? ‘When he kills her’ and thereby actively nullifies the positive commandment. And that is why he says: We have only… ”
And in s.v. “This works well” — “With regard to sending away the mother bird, you can find it if he nullified it ‘when he slaughtered her’; but here, how can you find it?”
It should be noted: why did Rashi first write that he nullified the positive commandment through killing, and afterward use the term slaughtering? This needs a bit of further examination.
Many thanks for everything,