How Can God's Mercy Be Also His Justice??!!

We read, "He is faithful and JUST to forgive us our sins  
And to cleanse us from all unrighteousness"(1 John  1.9).  
   
Wait a minute. I thought forgiveness = mercy,  
While justice means to meet out punishment for sin.  
So that mercy doesn't = justice    
and justice is distinct from mercy.  
   
So how can mercy = justice?    
It's wrong to punish someone severely  
if he has been entrapped,  
somebody who doesn't know what they're doing.  
   
In that case it is just to have mercy    
to the extent that it,the sin, isn't (all) their fault.  
THAT does describe the human condition to the extent that    
when a boy commits a robbery he isn't consciously opting for a life of crime,  
It's only a one-time thing to him.  
When a teenager commits a homosexual act,  
If he had known it would lead to a lifetime of bondage  
he may well have been stirred to reject that first act.  
   
Finally, one reason God's mercy is justice is this:  
God is supposed to fight evil, defeat the devil, etc, etc.  
But instead He lets robberies, rapes, murders occur.  
If a policeman stood idlely by while, in front of his face,    
a rape was taking place with the woman screaming, "Someone Help Me!!"  
Wouldn't that make him, the cop, "an accessory after the fact"?  
Likewise, God, the heavenly policeman,
is not in the position to dump all the blame on us,
FOR IT IS HE WHO LETS BAD THINGS HAPPEN.  
   
I mean He wants US to forgive Him for allowing stuff to happen,  
So it is only fair and reasonable for HIM to forgive us for our part in sin.  
THAT is why God is being "just" by forgiving our sins - for His own hands aren't perfectly clean !!!  
And that is, of course, why He (via the Son)
went to the Cross - to pay for the sins-of-omission by the Father.  
It's NOT that they were sins, rather that they WOULD HAVE BEEN sins
 - if Christ hadn't paid for them in advance.  
Thus, Christ's crucifixion not only saved us humans from our sins,  
It also saved GOD THE FATHER from being the biggest/worst sinner of all.
 
PS: Why do I say "paid for them in advance"? (3rd line from the end.)  ....
Because Christ's crucifixion was counted
from before the universe began (Revelation 13.8).
He was "the Lamb slain before the foundation of the world."

PPS: Here's confirmation that Christ paid for the deeds of the Father:
(1)"The reproaches of those that reproach YOU [the FATHER],
have fallen on Me [the Son]"(Psalms 69.9).
To paraphrase it, "The blame of those who blame God the Father  
has fallen on the God the Son."
 
(2)The sons of King Saul were hung  
(on a tree?) on "the mountain of the Lord"
As payment for the sins of their father,Saul,
Against the Gibeonites(2 Samuel 21.1-9).
What's especially noteworthy here is that it happened  
Right at "the beginning of the barley harvest"(21.9),
Which clearly means at Passover time,  
Just like Christ was nailed to a tree at Passover.