What does Shakespeare's use of euphemism in Macbeth's soliloquies show about Macbeth? Esp. in the soliloquy of Act 1, Scene 7

That is a very good answer!
I thought I might add an interesting point that merely conforms with what you have said as a sideline illustration.
In Act IV scene I the witches describe what they do as –a deed without a name-
In his soliloquies prior to Duncans murder Macbeth refers to the act in much the same way. He uses the word do rather than name the deed.
-If it were done, when tis done, then twere well / It were done quickly (I.vii.1-2)-
-I go and it is done (II.i.62)-
Done is a funny word in Shakespeare as it usually refers to sex, equalising the act of murder with consummation.
Elsewhere the deed is it… Macbeth speaks the forbidden word but once
-My thought, whose murther yet is still fantastical-
And this is equivocation, while admitting he is thinking of murder he also suggests that thought itself is murdered.
Throughout Macbeth dialogue dwells obsessively on the unnameable, both action and identity are lost in the confounded language of the play.

Read- Shakespeare: Violation and Identity, by Alexander Leggatt for more detail