To answer why G-d had to die on the cross that is simple, to fulfill Scripture the Scripture says Hebrews 6:13-8:
13 For when God made his promise to Avraham, he swore an oath to do what he had promised; and since there was no one greater than himself for him to swear by, he swore by himself 14 and said, "I will certainly bless you, and I will certainly give you many descendants"; 15 and so, after waiting patiently, Avraham saw the promise fulfilled. 16 Now people swear oaths by someone greater than themselves, and confirmation by an oath puts an end to all dispute. 17 Therefore, when God wanted to demonstrate still more convincingly the unchangeable character of his intentions to those who were to receive what he had promised, he added an oath to the promise; 18 so that through two unchangeable things, in neither of which God could lie, we, who have fled to take a firm hold on the hope set before us, would be strongly encouraged. 19 We have this hope as a sure and safe anchor for ourselves, a hope that goes right on through to what is inside the parokhet, 20 where a forerunner has entered on our behalf, namely, Yeshua, who has become a cohen gadol forever, to be compared with Malki-Tzedek. 1 This Malki-Tzedek, king of Shalem, a cohen of God Ha'Elyon, met Avraham on his way back from the slaughter of the kings and blessed him; 2 also Avraham gave him a tenth of everything. Now first of all, by translation of his name, he is "king of righteousness"; and then he is also king of Shalem, which means "king of peace." 3 There is no record of his father, mother, ancestry, birth or death; rather, like the Son of God, he continues as a cohen for all time. 4 Just think how great he was! Even the Patriarch Avraham gave him a tenth of the choicest spoils. 5 Now the descendants of Levi who became cohanim have a commandment in the Torah to take a tenth of the income of the people, that is, from their own brothers, despite the fact that they too are descended from Avraham. 6 But Malki-Tzedek, even though he was not descended from Levi, took a tenth from Avraham. Also, he blessed Avraham, the man who received God's promises; 7 and it is beyond all dispute that the one who blesses has higher status than the one who receives the blessing. 8 Moreover, in the case of the cohanim, the tenth is received by men who die; while in the case of Malki-Tzedek, it is received by someone who is testified to be still alive. 9 One might go even further and say that Levi, who himself receives tenths, paid a tenth through Avraham; 10 inasmuch as he was still in his ancestor Avraham's body when Malki-Tzedek met him. 11 Therefore, if it had been possible to reach the goal through the system of cohanim derived from Levi (since in connection with it, the people were given the Torah), what need would there have been for another, different kind of cohen, the one spoken of as to be compared with Malki-Tzedek and not to be compared with Aharon? 12 For if the system of cohanim is transformed, there must of necessity occur a transformation of Torah. 13 The one about whom these things are said belongs to another tribe, from which no one has ever served at the altar; 14 for everyone knows that our Lord arose out of Y'hudah, and that Moshe said nothing about this tribe when he spoke about cohanim. 15 It becomes even clearer if a "different kind of cohen," one like Malki-Tzedek, arises, 16 one who became a cohen not by virtue of a rule in the Torah concerning physical descent, but by virtue of the power of an indestructible life. 17 For it is stated, "You are a cohen FOREVER, to be compared with Malki-Tzedek." 18 Thus, on the one hand, the earlier rule is set aside because of its weakness and inefficacy 19 (for the Torah did not bring anything to the goal); and, on the other hand, a hope of something better is introduced, through which we are drawing near to God. 20 What is more, God swore an oath. For no oath was sworn in connection with those who become cohanim now; 21 but Yeshua became a cohen by the oath which God swore when he said to him, "ADONAI has sworn and will not change his mind, 'You are a cohen forever.'" 22 Also this shows how much better is the covenant of which Yeshua has become guarantor. 23 Moreover, the present cohanim are many in number, because they are prevented by death from continuing in office. 24 But because he lives forever, his position as cohen does not pass on to someone else; 25 and consequently, he is totally able to deliver those who approach God through him; since he is alive forever and thus forever able to intercede on their behalf. 26 This is the kind of cohen gadol that meets our need - holy, without evil, without stain, set apart from sinners and raised higher than the heavens; 27 one who does not have the daily necessity, like the other cohanim g'dolim, of offering up sacrifices first for their own sins and only then for those of the people; because he offered one sacrifice, once and for all, by offering up himself. 28 For the Torah appoints as cohanim g'dolim men who have weakness; but the text which speaks about the swearing of the oath, a text written later than the Torah, appoints a Son who has been brought to the goal forever. 1 Here is the whole point of what we have been saying: we do have just such a cohen gadol as has been described. And he does sit at the right hand of HaG'dulah in heaven.d 2 There he serves in the Holy Place, that is, in the true Tent of Meeting, the one erected not by human beings but by ADONAI. 3 For every cohen gadol is appointed to offer both gifts and sacrifices; so this cohen gadol too has to have something he can offer. 4 Now if he were on earth, he wouldn't be a cohen at all, since there already are cohanim offering the gifts required by the Torah. 5 But what they are serving is only a copy and shadow of the heavenly original; for when Moshe was about to erect the Tent, God warned him, "See to it that you make everything according to the pattern you were shown on the mountain." 6 But now the work Yeshua has been given to do is far superior to theirs, just as the covenant he mediates is better. For this covenant has been given as Torah on the basis of better promises. 7 Indeed, if the first covenant had not given ground for faultfinding, there would have been no need for a second one. 8 For God does find fault with the people when he says, "'See! The days are coming,' says ADONAI, 'when I will establish over the house of Isra'el and over the house of Y'hudah a new covenant. 9 "'It will not be like the covenant which I made with their fathers on the day when I took them by their hand and led them forth out of the land of Egypt; because they, for their part, did not remain faithful to my covenant; so I, for my part, stopped concerning myself with them,' says ADONAI. 10 "'For this is the covenant which I will make with the house of Isra'el after those days,' says ADONAI: 'I will put my Torah in their minds and write it on their hearts; I will be their God, and they will be my people. 11 "'None of them will teach his fellow-citizen or his brother, saying, "Know ADONAI!" For all will know me, from the least of them to the greatest, 12 because I will be merciful toward their wickednesses and remember their sins no more.'"f 13 By using the term, "new," he has made the first covenant "old"; and something being made old, something in the process of aging, is on its way to vanishing altogether.
The bottom-line is the life is in the blood which could only be satisfied by G-d – Yashua the perfect, sinless G-d, Man.