How were they Metamei My Mikdash?
Radak: Their Korbanos [to Me] are Tum'ah.
Why does it say "ba'Yom ha'Hu"?
Rashi: On the day that they slaughtered their children, they come in front of Me in My Mikdash to anger Me.
Radak: On the same day that they served idolatry, they come to offer in My house, as if I do not see their deeds.
Malbim: On the day that they offered their children to Molech, they went to the Mikdash in order to offer a Korban to profane the Mikdash.
Why does it mention Chilul Shabbos here?
Rashi: They slaughtered their children on Shabbos.
Malbim: They intentionally did so on Shabbos, in order to also desecrate Shabbos. The Chilul applied to the most Kadosh [place] in the world (the Mikdash), the most Kadosh [time] in the year (Shabbos), on the most Kadosh Nefesh (a son special for Hashem, to learn Torah)
