ואפילו העובר על איסור קל של דברי סופרים מקרי רשע כדאיתא בפ"ב דיבמות ובפ"ק דנדה,
Even he who violates a minor prohibition of the Rabbis is called wicked, as it is stated in Yevamot, ch. 2, and in Niddah, ch. I.
Transgression of a Rabbinic prohibition involving an action.
ואפילו מי שיש בידו למחות ולא מיחה נקרא רשע [בפרק ו' דשבועות],
Moreover, even he who has the opportunity to forewarn another against sinning and does not do so is called wicked (ch. 6, Shevuot).
Doesn't transgress himself, only doesn't act to prevent someone else from transgressing.
וכל שכן וקל וחומר במבטל איזו מצות עשה שאפשר לו לקיימה, כמו כל שאפשר לו לעסוק בתורה ואינו עוסק, שעליו דרשו רז"ל כי דבר ה' בזה וגו' הכרת תכרת וגו', ופשיטא דמקרי רשע טפי מעובר איסור דרבנן.
All the more so he who neglects any positive law which he is able to fulfil, for instance, whoever is able to study Torah and does not, regarding whom our Sages have quoted, "Because he hath despised the word of the Lord... [that soul] shall be utterly cut off...," It is thus plain that such a person is called wicked, more than he who violates a prohibition of the Rabbis.
Neglects a Torah command, does not transgress prohibition. No action involved. This is even worse than transgressing a Rabbinic prohibition.
Why the double language of "וקל וחומר" & "וכל שכן"?
Also, seemingly the "ופשיטא" at the end is repetitious because it is still referring to the stringency of neglecting a Torah precept over transgressing a Rabbinical prohibition.
דרש רבא מאי דכתיב (קהלת יב, יב) ויותר מהמה בני הזהר עשות ספרים הרבה וגו' בני הזהר בדברי סופרים יותר מדברי תורה שדברי תורה יש בהן עשה ולא תעשה ודברי סופרים כל העובר על דברי סופרים חייב מיתה
Rabba taught: This that it is written (Ecclesiastes 12:12) , "And more than these, my children, be wary - make many books" etc., implies; "My children be wary" - of the edicts of the Rabbis more than the edicts of the Torah. The edicts of the Torah carry the weight of a positive and a negative commandment yet those who transgress the edicts of the Rabbis are punishable with death.
Since the edicts of the Rabbis are more stringent than the edicts of the Torah the Term "וקל וחומר" can't be used in this regard because you can not learn out that one is a רשע for transgressing the less stringent Torah edict when someone who transgresses the more stringent Rabbinical edict is called a רשע.
Therefor the therm "וכל שכן" (a term of lesser strength) is used because the discussion is: when is a person referred to as a רשע (not which is more stringent, transgressing Torah edicts or Rabbinic edicts).
In this regard, the neglect of a Torah precept is worse than transgressing prohibitions of the Rabbis:
מתני' ואלו הן הפסולין המשחק בקובי' והמלוה בריבית ומפריחי יונים וסוחרי שביעית, וגו'
Mishnah: These are all disqualified from giving testimony; Those that gamble with dice, those that lend money with interest, those that race pigeons and those that sell crops of the Sabbatical year...
Rash Explains: all these are disqualified because their behavior is similar to theft and the Torah says: "Don't make a רשע a witness"
The Tosfos explains at length that the Mishnah is talking about those that are disqualified on Rabbinic grounds.
These sources show that one who transgresses a Rabbinic prohibition is called a רשע.
The transgressing of a Rabbinic prohibition results in a Rabbinic disqualification...
As opposed to transgressing a Torah prohibition results in a Torah disqualification.
From these sources we see that someone who transgresses a Torah prohibition is also disqualified from testimony because the are called a רשע.
Therefore the Alter Rebbe uses the term "וכל שכן" to inform us that just as someone who transgresses a Torah edict is called a רשע so too is someone who neglects to fulfill a edict of the Torah is also called a Rasha because of the verses that the Alter Rebbe brings in the text.
כל עבירות שבתורה נפרעין ממנו וכאן ממנו וממשפחתו...
...התם בדינא אחרינא...
...מלמד שכל ישראל ערבים זה בזה...
The punishment for all the sins in the Torah is exacted upon the transgressor and in this case (of making a false oath) it is exacted upon him and his family...
...There, the family has a different punishment exacted upon them...
...from this we learn that the entire Jewish people are guarantors for one another...
The Ran comments on "התם בדינא אחרינא" and says that the punishment for the family is light "קלה" in relation to the punishment exacted upon the transgressor himself. Yet it is still clear that they are being punish because there is somewhat of their own sin involved. So if we were to compare the sin of the sinner and the sin of their family as well as the punishment of the sinner and the punishment of the family we could call them Light and Harsh, "וקל וחומר".
התם שיש בידם למחות ולא מיחו...
That case is one where he had the opportunity to forewarn him against sinning and did not do so...
We see the idea of punishment being exacted in relation to "וקל וחומר" in the case of not warning someone against sin. This is why the use of the term "וקל וחומר" is used when comparing this case and the case of someone who doesn't preform a mitzvah that they are capable of performing.
Therefore at the end the seeming reppetition of transgressing a Rabbinic prohibition is referred to as "ופשיטא" this is any Rabbinic prohibition including having not prevented someone from doing a sin because that is included with the use of the "וקל וחומר" and not a וכל שכן alone.