Many different answers are given to the above question. For example, there are those with personal axes to grind who will tell you that it was the Romans or the Jews. Such answers are childish excuses of the sort ‘it wasn’t me, it was him’.
More generally it is true to say that fallen humanity killed Christ, or more exactly it was hatred, cruelty, ingratitude and indifference that killed Christ. In three words this general answer can even be summed up as a ‘lack of love’, for this is the greatest killer of all, for it kills individuals, parish churches, monasteries and indeed, as we are witnesses, whole dioceses. However, this lack of love has in human history taken two specific forms.
The first form of a lack of love is the error of putting creation above the Creator, in other words, idolatry. This is the error of paganism that we can see in animism, superstition and throughout paganism. The further away from Jerusalem, the more pagan people were.
This we can see in how the Creation story, faithfully recorded only in the Book of Genesis, became ever more distorted in the creation myths of Australia or the Americas, India or Africa, Scandinavia or China.
This misidentification was the error of the Romans who made their emperors into gods, putting Caesar above Christ. Through lack of love of God and man, creation, fallen and sinful, is identified as Divine. A fundamental error and distortion.
The second form of a lack of love is the error of putting oneself (and so love of oneself) above the Creator. This self-idolatry was the error of the Jewish elite, but it can be found among the ‘professionals’ of all institutional religion, which is always opposed to faith.
This second form is called phariseeism, which makes salvation most difficult for all except its own members, who are the self-appointed elect. This misidentification is the error of humanism, which deifies fallen human nature. Humanism has its origins in papism, which put one man above God, making God absent and so enabling itself to replace him.
And humanism is merely papism extended to all Western and Westernized humanity, putting the self above the Creator. Through lack of love of God and man, creation that is fallen and sinful is identified as above the Divine. A fundamental error and distortion.
Who killed Christ?
Human lack of love that put itself on the level of the Divine and, even worse, even above the Divine.
Although the Romans were guilty of the first form of lack of love, in their indifference ignoring Christ, and although the Jews were guilty of the second form, in their jealous hatred of the Truth (and pharisees hate nothing more than the Truth, for it shows them up as they are, as liars), all fallen humanity is capable of such acts.
The fact is that if Christ returned today, He would be killed again. Indeed, this is why when He returns, He will return ‘with power’ (Lk. 21, 27). Who killed Christ? Lack of love, for God is love, but sin is lack of love.