I think that the the question as it has been formulated is very interesting. This describes perfectly the difficulties to analyze a delicate situation.
I want to try to understand the situation, and to describe it from multiple halachic points of view.
. How to judge the sin, and its diverse degrees according to different parts of Shulchan Aruch.
. What is the right relationship with someone who is really punishable by death sentence, or at least who is really liable for a deliberate great sin.
I. Shabbat laws: See SA OC 316:
- An action prohibited on Shabbat, to be called "a work" defined as work scripturally, needs to be a work in which the result is the purpose of the worker. E.g to trap a murex with the intention to made blue dye.
- We need an action which is truelly linked to the result as to cut the head of a bird is linked to kill. See saif 4 Magen Avraham sk 9, if there is a little hole in the net and the moth can go out, it is allowed to trap it because he can exit.
- Even if he wants the moth, if this kind of moth is not usually trapped, there is no punishment scripturally spoken, (saif 3).The word Chayav says chayav chat'at (atonement sacrifice) or mita (death sentence), for a sin deorayta (scriptural) which is chayav chat'at on shogeg (needs an atonement sacrifice when the sin is made by error), karet (cut off) in mezid (intentionally) and eventually mita (death sentence) with Edim (witnesses) and hatraa (watching).
The rebbe did not need the moth, so there is no mita (death sentence) and no chat'at (atonement sacrifice). If the pool net is not very hermetic, because of great holes or because he is not closed on the ground, it is permitted.
Let's say that this is a classic case of work" of Shabbat.
An important detail of the question:
he said that it's ok and did it anyway.
He says that it's ok, so he thinks that it's permitted. So the chilul (profanation of) Shabbat is Shogeg (erroneous). He is still a good Jew, until you give him an understandable proof that he is wrong. Many times, for this reason, Gemara and poskim said: "מוטב יהיו שוגגים ולא מזידים". That is better that they do this by error than intentionally.
II. Testimony laws. From rules of testimony when the sin is intentional, SA CM 34.
1)
רשע פסול לעדות ואפי' עד כשר שיודע בחבירו שהוא רשע ואין הדיינים מכירים רשעו אסור לו להעיד עמו אע"פ שהוא עדות (לשון הרמב"ם פ"י מעדות) אמת ואצ"ל עד כשר שהוא יודע בעדות לחבירו וידע שהעד השני שעמו עד שקר שאסור לו להעיד:
A miscreant is unfit for testimony. Even one fit witness who knows that the second witness is a miscreant, when the judges don't know this, should not testimony with him, even if thethe testimony is true ...
2) איזהו רשע כל שעבר עבירה שחייבים עליה מלקות ואצ"ל אם חייבים עליה מיתת ב"ד ל"ש אם עבר לתיאבון ל"ש אם עבר להכעיס:
Who is called miscreant regarding this rule? Everyone who broke any rule punishable by death or whipping, without any distinction according to wether reason, if solely motivated by money or on strike to rebel.
III. Wine laws. SA YD 124, 8:
... A renegade ... makes th wine "nesech" (prohibited as if he was used for idolatry) by touching...
IV. Shechita laws. SA YD 2 (rules of shechita), 5:
... a renegade regarding. ... public Shabbat's profanation, ... is equivalent to an idolater (NJ)...
Baer Heytev sk 15 and 16:
בפרהסיא. היינו בפני עשרה אנשים ישראלים. גם צריך שיחלל כמה פעמים בפרהסיא אבל בפ"א לא.
Public (profanation), that is in the presence of ten Jewish men, and repeatedly, not one time only,...
משמע מדברי התוס' בעירובין דף ס"ט ע"א אפילו עובר עבירה שהיא איסור דרבנן מקרי מחלל שבת אם עובר בפני עשרה
ישראלים ועושה כן כמה פעמים בה"י ע"ש ועיין בפ"ח בק"א שחולק.
.... there is a controversy if when the rule of shabbat which is profanated is rabbinical, the rule is the same...
.......(טז) כעובד כוכבים: וכתב בש"ך דמומר להכעיס אפילו בפעם א' הוי כעובד כוכבים ושחיטתו נבלה מדינא. ואפילו נמצא סכינו יפה דמועד לנבלה בידים. רש"ל.
ועיין לקמן סימן קנ"ח דמומר להכעיס הוי כאפיקורוס ואפיקורוס שחיטתו נבלה ובמומר לעבודת כוכבים אפילו בצינעא אפילו רק פעם א' הוי מומר לכל התורה ודינו כעובד כוכבים וכן אם חילל שבת פעם א' בפרהסיא ... .
... see in siman 158, one is renegade on a strike to rebel, ... even in front of less than 10 Jewish men, ... even after one time is a renegate. Similarly, one who profanated Shabbat in front of ten Jewish men one time only is a renegate.
