In the Golden Calf massacre roughly 3000 people were killed:
"וַיַּעֲשׂוּ בְנֵי־לֵוִי כִּדְבַר מֹשֶׁה וַיִּפֹּל מִן־הָעָם בַּיּוֹם הַהוּא כִּשְׁלֹשֶׁת אַלְפֵי אִישׁ׃"
"The Levites did as Moses had bidden, and some three thousand of the people fell that day." Exodus.32.28
IIRC, all people donated jewelry and Aharon was the one to make the idol.
How were the executed 3000 Jews different in their behavior from the others? What exactly did they do with the Calf?
The Chizkuni, in his commentary on pasuk 28 writes that these 3.000 people needed to be killed because these 3.000 men intended that their conduct should be viewed as idolatrous:
כשלשת אלפי איש, “approximately three thousand men.” According to the plain meaning of the text, these three thousand men intended that their conduct should be viewed as idolatrous. This is why they had to be executed, just as the inhabitants of the idolatrous town in Deuteronomy chapter 14 have to be executed and their belongings have to be burned. (Based on Daat Baaley Tossaphot)
The Pirkei DeRabbi Eliezer (chapter 45:11) explains something similair to what the Chizkuni writes, e.g. that these people intended that their conduct should be viewed as idolatrous. The Pirkei DeRabbi Eliezer explains that all the people who kissed the golden calf with all his heart, his upper lip and his bones became golden:
He made Israel drink the water (with the dust of the calf). Everyone who had kissed the calf with all his heart, his upper lip and his bones became golden, and the tribe of Levi slew him, until there fell of Israel about three thousand men, as it is said, "And the sons of Levi did according to the word of Moses" (Ex. 32:28).
Note that the Pirkei DeRabbi Eliezer mentions the words "with all his heart", it seems to me that this supports the view of the Chizkuni, that their hearts were filled with the intention of worshipping the golden calf.
A really interesting approach to this is given by the Shelah in his commentary on this pasuk. He mentions that they (the 3.000 men) were already sentenced to death when they leaved Mitzrayim, however, their sentence was suspended temporarily. The episode of the sin with the golden calf was however when the sentence was fulfilled (Shenei Luchot HaBerit, Torah Shebikhtav, Ki Tisa, Torah Ohr):
Presumably, their guilt had been that they had already then harboured thoughts of an idolatrous nature, but had only found an outlet for these thoughts during the episode of the golden calf.
The Shelah here mentions the "thoughts of an idolatrous nature", as does the Chizkuni.
See Rashi to 32:20, from Yoma 66b:
אִם יֵשׁ עֵדִים וְהַתְרָאָה בְּסַיִף – כְּמִשְׁפַּט אַנְשֵׁי עִיר הַנִּדַּחַת שֶׁהֵן מְרֻבִּין – עֵדִים בְּלֹא הַתְרָאָה בְּמַגֵּפָה, שֶׁנֶּאֱמַר וַיִּגֹּף ה' אֶת הָעָם, לֹא עֵדִים וְלֹא הַתְרָאָה בְּהִדְרוֹקָן
If there were witnesses and a warning [that they worshipped the Calf], they were executed by the sword, like the citizens of an Ir Hanidachas when they're the majority. If there were witnesses but no warning, they were punished by a plague, as it says (v. 35), "G-d sent a plague upon the people." If there were neither witnesses nor a warning, they were punished with dropsy [like a sotah, as Rashi says earlier in the comment].
This is actually one of two opinions in the Gemara there. The other is that the distinction was between those who performed an act of worship (such as slaughtering or burning an offering to it), hugging and kissing it, or just being happy about what was going on.
