What does "Vayanichuhu" imply?
Rashi, from Toras Kohanim: This implies that he was isolated, even though this occurred at the same time as the Mekoshesh Etzim on Shabbos, who was also incarcerated until Moshe was told what to do. 1
Rashi: This was because unlike the Mekoshesh, whom Moshe knew was Chayav Misah, but he did not know which Misah he deserved (See Sifsei Chachamim), he did not know whether the Megadef was due to receive Misah at all.
QUESTIONS ON RASHI
Rashi writes that Moshe did not know whether the Megadef was due to receive Misah at all. Surely this is worse than one who curses a parent; Beis Din kills him!
Da'as Zekenim, Moshav Zekenim, Riva: Because it is more stringent, perhaps he does not get Misas Beis Din, so he will not get a Kaparah. This is why Beis Din does not kill one who hands over all his children to Molech. Rosh - Hashem pardoned His honor, and said that this will be no more stringent than one who curses a parent.
Hadar Zekenim: Ein Onshin Min ha'Din (we do not punish based on a Kal v'Chomer). Why do we need another verse to teach that Ein Onshin Min ha'Din? That verse was said before this Parshah. 1
Rashi writes that Moshe did not know whether the Megadef was due to receive Misah, and did not know how the Mekoshesh should be killed. R. Yehudah holds that Beis Din can kill only if he was warned which death he will receive!
Da'as Zekenim: Perhaps they warned them for all four Misos Beis Din; each accepted, and said 'I do so with intent to be killed via stoning, burning, the sword or choking!" However, this is difficult for the opinion that Safek warning is invalid. Must we say that there were four pairs of witnesses, and each gave Vadai warning for one of the Misos (e.g. one pair said 'you will surely be stoned for this
Rashi writes that the blasphemer was at the same time as the Mekoshesh Etzim, which was before Matan Torah. How can he say that he scoffed about Lechem ha'Panim? It was not made until they built the Mishkan almost a year after Matan Torah!
Riva, Moshav Zekenim: The Mitzvah of Lechem ha'Panim was already taught, even though they did not make it until the Mishkan was built.
Moshav Zekenim citing the Ri: The text [in Toras Kohanim] said that the Mekoshesh was b'Shabbos Shel Shanah Sheniyah; the scribe erred and wrote 'Shabbos Sheniyah. 1
If so, we must also change the Beraisa to say that had Yisrael observed Shabbos the entire first year