Is Complaining a Sin?

Complaining is serious as far as God is concerned

1 Complaining About Hardships
-Their complaining displeased the Lord
-Why?
-What was their punishment?

2 Complaining About Food
-Craving for flesh
-They loathed God's mana
-What did God do

3 Moses Own Family Complained About Him
-They were complaining about God's chosen leader
-What did God do

What Can We Learn From This