Objection 1. It would seem that Christ was not slain by another, but by Himself. For He says Himself (John 10:18): "No men taketh My life from Me, but I lay it down of Myself." But he is said to kill another who takes away his life. Consequently, Christ was not slain by others, but by Himself.
Objection 2. Further, those slain by others sink gradually from exhausted nature, and this is strikingly apparent in the crucified: for, as Augustine says (De Trin. iv): "Those who were crucified were tormented with a lingering death." But this did not happen in Christ's case, since "crying out, with a loud voice, He yielded up the ghost" (Matthew 27:50). Therefore Christ was not slain by others, but by Himself.
Objection 3. Further, those slain by others suffer a violent death, and hence die unwillingly, because violent is opposed to voluntary. But Augustine says (De Trin. iv): "Christ's spirit did not quit the flesh unwillingly, but because He willed it, when He willed it, and as He willed it." Consequently Christ was not slain by others, but by Himself.
On the contrary, It is written (Luke 18:33): "After they have scourged Him, they will put him to death."
I answer that, A thing may cause an effect in two ways: in the first instance by acting directly so as to produce the effect; and in this manner Christ's persecutors slew Him because they inflicted on Him what was a sufficient cause of death, and with the intention of slaying Him, and the effect followed, since death resulted from that cause. In another way someone causes an effect indirectly--that is, by not preventing it when he can do so; just as one person is said to drench another by not closing the window through which the shower is entering: and in this way Christ was the cause of His own Passion and death. For He could have prevented His Passion and death. Firstly, by holding His enemies in check, so that they would not have been eager to slay Him, or would have been powerless to do so. Secondly, because His spirit had the power of preserving His fleshly nature from the infliction of any injury; and Christ's soul had this power, because it was united in unity of person with the Divine Word, as Augustine says (De Trin. iv). Therefore, since Christ's soul did not repel the injury inflicted on His body, but willed His corporeal nature to succumb to such injury, He is said to have laid down His life, or to have died voluntarily.
Reply to Objection 1. When we hear the words, "No man taketh away My life from Me," we must understand "against My will": for that is properly said to be "taken away" which one takes from someone who is unwilling and unable to resist.
Reply to Objection 2. In order for Christ to show that the Passion inflicted by violence did not take away His life, He preserved the strength of His bodily nature, so that at the last moment He was able to cry out with a loud voice: and hence His death should be computed among His other miracles. Accordingly it is written (Mark 15:39): "And the centurion who stood over against Him, seeing that crying out in this manner, He had given up the ghost, said: Indeed, this man was the Son of God." It was also a subject of wonder in Christ's death that He died sooner than the others who were tormented with the same suffering. Hence John says (19:32) that "they broke the legs of the first, and of the other that was crucified with Him," that they might die more speedily; "but after they were come to Jesus, when they saw that He was already dead, they did not break His legs." Mark also states (15:44) that "Pilate wondered that He should be already dead." For as of His own will His bodily nature kept its vigor to the end, so likewise, when He willed, He suddenly succumbed to the injury inflicted.
Reply to Objection 3. Christ at the same time suffered violence in order to die, and died, nevertheless, voluntarily; because violence was inflicted on His body, which, however, prevailed over His body only so far as He willed it.
Source: Summa Theologica (New Advent)