Why The Blood?

There is the idea that "the life is in the blood".
I am not sure how far we can extrapolate into this thought, but we see the blood Abel crying out to God for justice. Whether this is literal or poetic... who can tell.
But we almost always we see "blood" and "life" and "justice" mentioned together.

2 Kings 9:
26 Surely I have seen yesterday the blood of Naboth, and the blood of his sons, saith the LORD; and I will requite thee in this plat, saith the LORD. Now therefore take [and] cast him into the plat [of ground], according to the word of the LORD.

Job 16:
18 O earth, cover not thou my blood, and let my cry have no place.

Psalms 9:
12 When he maketh inquisition for blood, he remembereth them: he forgetteth not the cry of the humble.

Psalms 72:
14 He shall redeem their soul from deceit and violence: and precious shall their blood be in his sight.

In Hebrews we see the blood opened up:

Hebrews 12:
24 And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than [that of] Abel.
25 See that ye refuse not him that speaketh. For if they escaped not who refused him that spake on earth, much more [shall not] we [escape], if we turn away from him that [speaketh] from heaven:

I believe these verses are contrasting the blood of Abel that cried to God for justice against Cain - and the blood of Jesus that also cries for justice in favor of we those are benefactors of redemption. The blood of Jesus cries, not from earth like the blood of Abel, but from heaven; where he took it to be placed on the alter. Unlike the blood of Abel which cried for vengence against Cain, the blood of Jesus cries for vengence against the murderer of the spirit of Adam and all mankind. The blood of Jesus, who became a part of that victim when he was made sin for us, now cries for justice against the murderer of mankind: the devil! So this is where we have the power of the blood. The blood speaks, and the blood of Jesus is the basis for our power, authority, and vergence against the devil... the murderer of our souls!

So we see the victory over the devil with the blood of the lamb:

Revelation 12:
9 The huge dragon was thrown down. That ancient serpent, called devil and Satan, the deceiver of the whole world, was thrown down to the earth, and its angels were thrown down with it.
10 Then I heard a loud voice in heaven say, "Now the salvation, the power, the kingdom of our God, and the authority of his Christ have come. For the one accusing our brothers, the one who accuses them day and night in the presence of our God, has been thrown out.
11 They conquered him by the blood of the lamb and by the word of their testimony, for they did not love their life even in the face of death.

The blood covenant is a great topic. There are a few good books. I know E.W. Kenyon did a mini book on the topic.