What is the meaning of "Chata'im ha'Eileh be'Nafshosam"?
Rashi: It means that they were sinners in that they by quarreling with Hashem, they forfeited their lives.
Da'as Zekenim (16:7), Riva (16:6, citing R"M of Kutzi): They were warned [that they will die], and did not listen.
Moshav Zekenim: They are liable for themselves. It says about one who denied himself wine "Chata Al ha'Nafesh" (6:11), and all the more so these, who caused their own death!
What are "Riku'ei Pachim"?
What sort of sign did the copper plates display?
Rashi: They served as a reminder that they belonged to the people who had quarreled against the Kehunah and had been burned.