Why did God relent on killing Moses when Zipporah circumcised their son? In Exodus 4 we are told that God sought to kill Moses for not circumcising his son in accordance with the law given to Abraham in:
Genesis 17:11 KJV

And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you.

but he relented on killing Moses when Moses' wife Zipporah took a sharp stone and circumcised the child, even though Moses himself did not circumcise the boy.
Exodus  4:24  through 26 KJV

24And it came to pass by the way in the inn, that the LORD met him, and sought to kill him.
25Then Zipporah took a sharp stone, and cut off the foreskin of her son, and cast it at his feet, and said, Surely a bloody husband art thou to me.
26So he let him go: then she said, A bloody husband thou art, because of the circumcision.

It seems strange that God relented with Moses and yet killed the men of Bethshemesh:
1 Samuel 6:19  KJV

And he smote the men of Bethshemesh, because they had looked into the ark of the LORD, even he smote of
the people fifty thousand and threescore and ten men: and the people lamented, because the LORD had
smitten many of the people with a great slaughter.

and Uzza for using his hand to keep the Ark from falling off thee cart:
1Ch 13:9 &amp; 10  KJV

9And when they came unto the threshingfloor of Chidon, Uzza put forth his hand to hold the ark; for the oxen stumbled.
10And the anger of the LORD was kindled against Uzza, and he smote him, because he put his hand to the
ark: and there he died before God.

While I full well understand that God does not need to explain his reasons, it would seem that a just God would treat both Moses and Uzza the same.
God must have had his reasons for doing it this way; are there Biblical reasons for believing his behavior in this case was consistent with the other behavior I've mentioned?
The Bible "stories" teach us about the nature of God.  They sometimes serve as a warning and other times they encourage a closer relationship with God. 

These stories are not casual happenings; they are recorded in the Bible as an everlasting message to God’s children.  So in the case of Uzza, the Ark is the presence of God on Earth.  When Uzza reached out to save the Ark this is in complete contrast to the “truth”, God saves man, man does not save God.  So while this seems harsh and Uzza may have reacted out of instinct the eternal-stakes were much greater than Uzza’s life.  We also must realize that just because God took Uzza’s life it does not follow that Uzza went to Hell, but may have gone to be with the Lord. 

Once Moses’ son was circumcised God no longer sought to kill Moses.  God instructs Moses to tell Pharaoh that Israel is God’s first-born son.  Israel ultimately bore the “only begotten Son” Jesus Christ.  Obviously Moses’ neglect regarding his own son was inexcusable and compromised the validity of God’s full message to humanity.  God will not compromise; He is not tolerant but patient (long-suffering), which is an attribute of God’s love.

The same is true with the people of Beth Shemesh that compromised these sacred articles of God, by looking into the Ark of the Covenant.  All of these stories send a message even to this day.