Analyze Macbeth's line, "We have scorched the snake not killed it." How does this represent guilt in Macbeth?

Macbeth speaks this line just after Lady Macbeth has advised him not to think about things he cannot change.  She asks why he keeps to himself, with only his sad thoughts as his companions, saying, "Things without all remedy / Should be without regard.  What's done is done" (3.2.13-14).  In other words, she tells him that he needs to move on.  If he cannot change or fix something in the past, then there is really no reason to dwell on it.

However, Macbeth suggests that there are more things that need to be done when he says, 

We have scorched the snake, not killed it.
She'll close and be herself whilst our poor malice
Remains in danger of her former tooth.
But let the frame of things disjoint, both the worlds
     suffer,
Ere we will eat our meal in fear, and sleep
In the affliction of these terrible dreams
That shake us nightly.  (3.2.15-22)

What he means is that they have injured the snake, but not ridden themselves of it completely.  It will heal and return to hurt them. Macbeth is trying to stay strong, but his guilt seems to be responsible for causing him to lose sleep (something he predicted would happen right after he committed the murder of Duncan). When he talks about the "snake," he's really using it as a metaphor for anything or anyone that would endanger their crown.  

Further, he continues, saying, 

Better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy.  Duncan is in his grave.
After life's fitful fever he sleeps well.
Treason has done his worst; nor steel no poison,
Malice domestic, foreign levy, nothing
Can touch him further.  (3.2.22-29)

Macbeth elaborates more on his guilt here.  He says that it would be better to be among the dead, who to make themselves feel at peace he and Lady Macbeth have sent to their peace (because they are resting in peace as dead persons), than to continue living with a tortured mind. Duncan is dead, and he sleeps peacefully now. The worst that anyone could do to him, they have done. And now he is immune to any other kind of harm. Macbeth, ironically, envies Duncan. Macbeth's guilt must indeed be extreme if he is now beginning to feel jealous of the very people he has killed because they, at least, are without guilt and he cannot be.