Salaam.
(1) That they killed Jesus, was more their proud boast, than reality, of which they themselves were not sure.
Their utter pride and prejudice is found in their mention of Jesus as Messenger of God as in 4:157, which please see.
All their ire at Jesus was only because he claimed that he was Messenger of God, and they rejected his claim. Yet they boasted that it was the Messenger of God that they killed.
(2) When they had stooped so low as to seek the death of God's Prophets, it was as though they were guilty of the most heinous crime of Prophets' murder. Because, the Prophets were saved only because of special divine protection. Their enemies were hell bent on killing them.
It is like Abraham actually trying to sacrifice his son, and God declaring: "...O Abraham, you did fulfill the dream..." (37:104-105) Although his son was divinely prevented from being killed!
(3) The Arabic derivative of the root QTL translated as killed, also means, to spoil, or utterly spoil some one, or to prevent or bar some one's success or growth.
Thus it will mean the Jews' most wicked and strongest efforts to spoil the Prophets' holy plans. It is as good as killing the holy Prophets, for all practical purposes.
Narrating the Ten Commandments, it is mentioned: "...and kill not your children for poverty; We provide for you and them..." (6:151).
Here again, the root is QTL. Here, most traditional commentators of Al Qur'an have stressed the point that it is the neglect of children - by not providing for their basic education, and thus spoiling them in the name of poverty - that is actually meant in (6:151). Qur'anists too have stressed the point, and they say that in the Arabia of Muhammed's time, there is no historic evidence that male children were killed. Qur'an too mentions only the killing of female children, elsewhere in Al Qur'an.
Regards,
A. Ismail Sait.