In Macbeth, did Macbeth ever feel guilty about the atrocities he committed, or did he just regret that his quest for power was unsuccessful?

I think that if we look carefully at the text, Macbeth,
even when it is clear that the game is up and he is going to be defeated, never displays
repentance or regret for his actions. The closest perhaps he gets to it comes when he
faces Macduff and says to him:

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But get thee back, my soul is too much
charg'd

With blood of thine
already.

Here, we see that
Macbeth obviously doesn't want to face Macduff because he has killed so many of his
family already and does not want to add another to the list. However, this is the
closest comment we see indicating any form of regret in what he
says.

As regarding your second possibility, that Macbeth is
only sad that his quest for power was unsuccessful, I am not necessarily sure that we
can find evidence to support this view either. To be honest, until his last appearance
in the play, Macbeth does not show much regret about anything. Even when it is clear
that all of the prophecies heralding his death have come to pass, he still chooses to
fight his way to death, unyielding to the end. Note what he
says:

I will
not yield,

To kiss the ground before young Malcolm's
feet,

And to be baited with the rabble's
curse.

Though Birnam wood be come to
Dunsinane,

And thou oppos'd, being of no woman
born,

Yet I will try the last: before my
body

I throw my warlike shield: lay on,
Macduff;

And damn'd be him that first cries, "Hold,
enough!"

Whatever we think of
Macbeth, we certainly have to appreciate his defiance and bravery up until the very end
in the face of certain defeat. He will "try the last," never letting himself give in or
become weak in the face of the witch's prophecies. He displays no regret that his bid
for power was ultimately unsuccessful, only a defiant spirit that carries him through to
his death.