But the final and decisive reason for the passion [Christ's] lay in man's sinfulness.  In His body Christ had to pay the price for the sin of men by submitting to death.  "What the law could not do, in that the flesh rendered it powerless, God sending His own Son, with a flesh like to that of sin, and in view of sin, has condemned sin in the flesh" (Rom., 8:3). The flesh of Christ was like to the flesh of sinful man precisely because it was not yet within the sanctuary; it was unglorified, subject to death; and death, of the body as well as of the soul, is the consequence of sin.  The obedience of Christ's will had to be expressed in His flesh, accepting death, the penalty of sin, if men were to be released from sin.  The moment on Calvary was the supreme moment only because then the full implications of interior sacrifice in the context of sinful humanity were accepted and fulfilled.  When Christ's body is raised from death mankind has the certainty that death has been overcome, that the power of sin has been broken.  And yet, as St. Thomas does not fail to insist, man's sin could have been absolved in other ways.  Without violating the law of justice, God could have condoned man's offence by an act of mercy.  In any event His mercy stands behind Christ's reparation for it was He who sent His Son, and this implies a basic condonation of sin.  The true sense of Christ's sufferings, the need for His death, are to be sought in man.  It was because God respects the dignity of the creature whom He has made to His own image that He sent His Son to die as man.  It was man who was overcome by Satan and man, if he is to bear the responsibilities of his freedom, must pay in full the consequence of his fault.  Because Christ suffered man can claim that humanity has met the debt of justice incurred by sin, an offence against God.

-- excerpted from Colman O'Neill's Meeting Christ in the Sacraments (p.34)

My bottom line: Thus, God is not a wrathful God who demands the death of His own Son in order to appease His own justice!