וַיָּ֥רֶב הָעָ֖ם עִם־מֹשֶׁ֑ה וַיֹּאמְר֣וּ לֵאמֹ֔ר וְל֥וּ גָוַ֛עְנוּ בִּגְוַ֥ע אַחֵ֖ינוּ לִפְנֵ֥י יְהוָֽה׃
И люди с Моисеем стремились и говорили: 'Если бы мы погибли, когда наши братья погибли перед Господом!
Sforno on Numbers
וירב העם עם משה, the subject of the quarrel was why Moses had brought the people to such a desert as the one they found themselves in at the time. However, there was also a complaint against G’d Himself, as the Torah testifies in verse 13 where the Torah writes: אשר רבו בני ישראל את ה', “in that the Children of Israel quarreled with G’d.” This referred to the words (verse 5) “why did you take us out of Egypt?”
Or HaChaim on Numbers
וירב העם עם משה, The people quarrelled with Moses. The people now complained that they would have preferred for Moses to have let them die from the plague than to face death by thirst.