One could say the roman with the spear killed him.
Or they could say any number of other people (ie a conspiracy).
At any rate, he came here with the intent of the outcome, so perhaps the question is not who killed Jesus, but who helped him fulfill his mission?
Actually, He was already dead, the soldier couldn't have killed Him:
John 19:32-24:
So the soldiers came, and broke the legs of the first man and of the other who was crucified with Him; but coming to Jesus, when they saw that He was already dead, they did not break His legs. But one of the soldiers pierced His side with a spear, and immediately blood and water came out.