What does "That had a heart to love and in that heart / Courage to make 's love known" mean?

The line
is spoken byin act 2, scene 3. As always with quotations, it's important to understand the
context in which they are spoken.

What's happening in this scene is that 's
brutal murder has just been uncovered. Everyone naturally assumes that it was his guards, whose
hands and daggers are covered in blood. They too are dead, andclaims responsibility for killing
them (but not for killing Duncan).

Whenasks him why he killed the guards,
Macbeth launches into a lengthy justification of his actions. He claims that his love for Duncan
was so powerful that it drove him into a violent rage before he'd had time to think through the
consequences of his actions.

Most of those around Macbeth would much rather
that he had not killed the guards. If they'd still been alive, then it would've been possible to
interrogate them and find out who sent them to kill Duncan and why. But of course Macbeth
doesn't want anyone to know the terrible truth of what really happened, so he killed the guards
to keep them quiet.

With an astonishing degree of cynicism and hypocrisy,
Macbeth asks those gathered round him a :

Who could
refrain,

That had a heart to love, and in that heart

Courage to make 's love known?

In other
words, how could anyone who loved Duncan have possibly restrained himself under such
circumstances? Macbeth made his "love known" by brutally slaying the men he wants
everyone to think were Duncan's assassins.