The people had rejected God. Their impatience and distrust, lack of faith caused them to question God and why they were having to wait on Moses and possibly there would be jealousy there too. Aaron, could be Jealous of Moses why else would he react the way he did. Aaron should have encouraged the people to wait, tried to strengthen their faith, instead of secumbing to their whims. People forget very easily what God does for them.
God had every right to be angry. I believe he would be sad too, disappointed, have a feeling of being let down, the people he loved had treated Him so shabbily. How do we react when a loved one treats us shabbily. We want to relatiate. Is this a characteristic of God since we are made in God's image?
Not only is God a God of love but He is a God of justice. Should the people truly repent He will forgive them