They got what they deserved is a satisfying way to bring something to closure. If he or she got what they deserved – their comeuppance, or some appropriate reward, then that means things turned out as they should, we’re good with that. On the other hand if you didn’t get what you deserved then that’s certainly good reason to complain.
This sense of right and wrong: what we deserve, what’s due to us, what’s mine by right – that’s something that runs very deep in us.
The big problem however is that God doesn’t see things that way. Paul points out to the Romans that God loves us not because we deserve it, but despite the fact that we don’t deserve it (Romans 5:6-11). Jesus built his church, sending his disciples into the world, because he recognized people were lost and confused (Matthew 9:36—10:8). It was a home for the troubled and abandoned, not the powerful and successful, those who deserved it.
And those early Israelites, that Moses complained about so vehemently (a stiff-necked people; wicked and sinful), they certainly didn’t deserve the promise that God gave them, that they should be his “special possession” (Exodus 19:2-6a).
We should be very grateful that God doesn’t treat us as we deserve. Maybe that should also make us less willing to call for consequences for others, based on what we think that they deserve. As Jesus noted to his disciples when he sent them as founding members of his church: “Without cost you have received; without cost you are to give.” The same applies to us.