What is the connection between the current Pesukim?
Seforno: Now that you see that Hashem is prepared to send you into exile in the event that you sin, 1 take care to observe His Mitzvos without adding to or subtracting from them, not even if the reason behind the Mitzvah no longer applies. 2 Because the slightest deviation from the Mitzvos brings destruction in its wake - like what happened by Ba'al Pe'or, where the men intended only to be intimate 3 with the Midyanite women without worshipping their idols. 4 But in the end, everyone who began with Z'nus ended up serving idolatry, and was wiped out.
Divrei Eliyahu: The Torah said "Al Tifnu el ha'Elilim" - 'Do not turn to idols (at all)'. Some of those who served Ba'al Pe'or wanted to add and disgrace it through sullying it with excrement, thinking that it was a great Mitzvah - "va'Yitzamed Yisrael le'Va'al Pe'or" (Balak Bamidbar, 25:3). Nevertheless, they were Chayav Misah, since that is its Avodah. The Mitzvah to disgrace idols iss confined to idolatry that is worshipped honorably. 5
The Torah places the Isur of adding to Mitzvos next to Ba'al Pe'or since initially it was served honorably. Its followers were fanatic, and served it even in the privy. Later generations did not worship it so passionately, just they persisted in their habit of serving it in the privy, and this became its only service, (Heard from R. Uziel Milevsky.)
Seforno (citing Sanhedrin, 21b): Like Sh'lomoh ha'Melech said 'I will add [extra wives] and I will not sin ... !'; but he did
Seforno: Which is prohibited mainly in order to avoid worshipping their idols.
Seforno: As the Pasuk wrote in Balak Bamidbar, 25:1.
We see that one may not rely on one's intellect to add or detract from Mitzvos, even le'Shem Shamayim!
