Objection 1. It would seem that there was some other more suitable way of delivering the human race besides Christ's Passion. For nature in its operation imitates the Divine work, since it is moved and regulated by God. But nature never employs two agents where one will suffice. Therefore, since God could have liberated mankind solely by His Divine will, it does not seem fitting that Christ's Passion should have been added for the deliverance of the human race.
Objection 2. Further, natural actions are more suitably performed than deeds of violence, because violence is "a severance or lapse from what is according to nature," as is said in De Coelo ii. But Christ's Passion brought about His death by violence. Therefore it would have been more appropriate had Christ died a natural death rather than suffer for man's deliverance.
Objection 3. Further, it seems most fitting that whatsoever keeps something unjustly and by violence, should be deprived of it by some superior power; hence Isaias says (52:3): "You were sold gratis, and you shall be redeemed without money." But the devil possessed no right over man, whom he had deceived by guile, and whom he held subject in servitude by a sort of violence. Therefore it seems most suitable that Christ should have despoiled the devil solely by His power and without the Passion.
On the contrary, St. Augustine says (De Trin. xiii): "There was no other more suitable way of healing our misery" than by the Passion of Christ.
I answer that, Among means to an end that one is the more suitable whereby the various concurring means employed are themselves helpful to such end. But in this that man was delivered by Christ's Passion, many other things besides deliverance from sin concurred for man's salvation. In the first place, man knows thereby how much God loves him, and is thereby stirred to love Him in return, and herein lies the perfection of human salvation; hence the Apostle says (Romans 5:8): "God commendeth His charity towards us; for when as yet we were sinners... Christ died for us." Secondly, because thereby He set us an example of obedience, humility, constancy, justice, and the other virtues displayed in the Passion, which are requisite for man's salvation. Hence it is written (1 Peter 2:21): "Christ also suffered for us, leaving you an example that you should follow in His steps." Thirdly, because Christ by His Passion not only delivered man from sin, but also merited justifying grace for him and the glory of bliss, as shall be shown later (48, 1; 49, 1, 5). Fourthly, because by this man is all the more bound to refrain from sin, according to 1 Corinthians 6:20: "You are bought with a great price: glorify and bear God in your body." Fifthly, because it redounded to man's greater dignity, that as man was overcome and deceived by the devil, so also it should be a man that should overthrow the devil; and as man deserved death, so a man by dying should vanquish death. Hence it is written (1 Corinthians 15:57): "Thanks be to God who hath given us the victory through our Lord Jesus Christ." It was accordingly more fitting that we should be delivered by Christ's Passion than simply by God's good-will.
Reply to Objection 1. Even nature uses several means to one intent, in order to do something more fittingly: as two eyes for seeing; and the same can be observed in other matters.
Reply to Objection 2. As Chrysostom [Athanasius, Orat. De Incarn. Verb.] says: "Christ had come in order to destroy death, not His own, (for since He is life itself, death could not be His), but men's death. Hence it was not by reason of His being bound to die that He laid His body aside, but because the death He endured was inflicted on Him by men. But even if His body had sickened and dissolved in the sight of all men, it was not befitting Him who healed the infirmities of others to have his own body afflicted with the same. And even had He laid His body aside without any sickness, and had then appeared, men would not have believed Him when He spoke of His resurrection. For how could Christ's victory over death appear, unless He endured it in the sight of all men, and so proved that death was vanquished by the incorruption of His body?"
Reply to Objection 3. Although the devil assailed man unjustly, nevertheless, on account of sin, man was justly left by God under the devil's bondage. And therefore it was fitting that through justice man should be delivered from the devil's bondage by Christ making satisfaction on his behalf in the Passion. This was also a fitting means of overthrowing the pride of the devil, "who is a deserter from justice, and covetous of sway"; in that Christ "should vanquish him and deliver man, not merely by the power of His Godhead, but likewise by the justice and lowliness of the Passion," as Augustine says (De Trin. xiii).
Source: Summa Theologica (New Advent)