Why is the prohibition of eating blood so severe that it causes G-d to ''make Himself free'' from all his affairs, and deal with the guilty person? And why does Rashi not address this obvious question?
Rashi did not need to explain why the prohibition of eating blood is so severe, because the Torah states the reason explicitly: ''Because the soul of [every creature's] body [depends on its] blood''.
Furthermore, the reader will remember that, after the Flood, G-d told Noach: ''Every moving thing that lives shall be yours to eat. Like the green vegetation [which was all that man could eat before] I have [now] given you everything'' [Noach 9:3]. Rashi [ibid] comments: ''I did not permit Adam, the first man, to eat meat, but only vegetation. But, for you, like the green vegetation which I allowed Adam [before], I have [now] given you everything''.
Why did G-d forbid Adam to eat meat and then permit it to Noach?
Rashi did not explain this matter as he held it to be self-evident. G-d forbade Adam to take the soul from a living creature merely for the sake of eating it. But after the Flood, there was a weakening of the physical makeup of man requiring the additional nutritional value of meat, and therefore G-d permitted man to eat meat.
Nevertheless, even after G-d permitted man to eat meat, He imposed certain restrictions. To non-Jews He prohibited eating meat which had been detached from a living animal [Noach 9:4] and to Jews he also prohibited the consumption of the animal's blood. For while a dispensation had been granted to eat meat, it was nevertheless not absolute. So, while it became necessary [for nutritional reasons] to allow man to eat the flesh of the animal, it remained prohibited to eat its blood, which contains the very life and soul of an animal.
Based on Sichos Shabbos Parshas Acharei 5746 - Lubavitcher Rebbe