The Parsha of Ki Tisa, especially "sheni", the second reading, is one of the most difficult in the Torah – not just the deep meaning, but it's actual content; the only time G'd suggests to Moshe
וְעַתָּה הַנִּיחָה לִּי וְיִחַר אַפִּי בָהֶם וַאֲכַלֵּם וְאֶעֱשֶׂה אוֹתְךָ לְגוֹי גָּדוֹל
Now leave Me alone, and My anger will be kindled against them so that I will annihilate them, and I will make you into a great nation.
The people are still next to Mount Sinai, still under the impression of the thunder and lightning – but perform a grave sin "As a bride being unfaithful under her chuppa" . G'd says to Moshe "Enough is Enough, I understand who I am dealing with, and want to put an end to it". The story is well known – Moshe comes down from the mountain, smashes the tablets, Shouts " Whoever is for the Lord, [let him come] to me!", and the tribe of Levi join him. Three thousand are killed by The Levi'im, and G'd adds more