What exactly, was their complaint?
Rashi (to Devarim 1:1): They complained that, if they were destined to die, then it would have been preferable to die during the three days of darkness (together with the four-fifths who died then), when they would at least have merited a decent burial; rather than to die in the desert, where their corpses would lie exposed to the heat by day and the cold by night. 1
Seforno: They complained that even if Pharaoh and his army would not attack them, now that they were camped behind them they were cut off from all sources of sustenance, and were destined to die in the desert - 'of hunger, thirst, nakedness and for want of all basic needs.'
QUESTIONS ON RASHI
Rashi writes: "'Ha'mi'Beli Ein Kevarim' (lit., 'is it because there are not no graves') - Is it because of a lack of graves; that there are no graves in Egypt in which to be buried, etc." Rashi is clearly bothered by the double negative. But how does he explain it?
Gur Aryeh: Rashi splits their rhetorical question into two parts - a. What was there no lack of? 'Is it because of a scarcity in [terms of] graves?' b. Where was there this supposed lack? '... that there are no graves in Egypt....'
