After the fight with Korach has been concluded, then a plague ran through the camp killing people, and Hashem stopped the plague, in 17:6 the passuk tells us how the people complained that Moshe Aand Aharon were killing off Gods nation, which leads to Hashem offering to Moshe that if he has had enough of them yet, He (God) would be willing to kill them all. He (God) is just waiting for the word from Moshe.
It is amazing how the people just keep on complaining. A group of people just complained a short while ago. They forced a confrontation, and even after Moshe warned them off they kept going, they all got killed with unusual deaths.
The past few Torah portions have given us other examples of the people's complaints and how Hashem dealt with them, often resulting in the deaths of the complainers.
The people complain and get killed. So how do they have the gall to complain yet again, especially after they just witnessed the horrific deaths of Korach and his group of supporters?
People like to complain. A leader has to know that before he takes the position as leader - he has to know that no matter what he does, no matter how much integrity he has, no matter how honest he is, the people he is leading will complain and kvetch.