In Macbeth, in what ways do the murderers' attitudes towards killing contrasts with Macbeth's earlier attitude towards the killing of Duncan?

In Shakespeare's Macbeth, the
murderers Macbeth uses to kill Banquo are unemotional about and unconcerned with what
they do.  They don't care if what they do is right or
wrong.

The Second Murderer
says:

I am
one, my liege,

Whom the vile blows and buffets of the world
[he's been kicked around, figuratively; the world has abused
him]

Have so incensed [made him angry with the world] that
I am reckless what

I do to spite the world.  (Act
3.1.108-111)

He's been so
abused by the world, he says, that he doesn't worry about what he does to the world.  He
doesn't care if what he does is wrong or right.

The First
Murderer has a similar attitude:

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And I another

So
weary with disasters, tugged with fortune [he's had bad
luck],

That I would set my life on any chance [make any
bet],

To mend it or be rid on't [he'd make any bet to
either drastically improve his life or end it, one or the other].  (Act
1.3.111-114)

In contrast,
Macbeth is very concerned about right and wrong when he is deciding whether or not to
assassinate Duncan.  In Act 1.7.1-28, Macbeth worries about the consequences of his
actions both on earth and in the afterlife.  He is bothered by Duncan's being a fair and
humble king.  He is bothered by the good treatment he has received from Duncan.  He is
bothered because Duncan is his guest, and a man should protect his guest, not
assassinate him.  Macbeth is concerned with issues of right and wrong, justice and
injustice, in contrast with the murderers he hires to kill Banquo, who
aren't.

Unfortunately, though Macbeth is aware, he still
ultimately decides to go ahead with the assassination.  But he's aware of the issue
involved with his final decision, too--the issue of his great
ambition. 

Ironically, a similarity exists between the two
scenes that are involved in answering your questions.  Though Macbeth seems resentful of
his wife's role in convincing him to go ahead with killing Duncan (or, at the least, he
shuts her out of all decision making from that point on), he uses the same methods in
convincing the murderers to do his bidding.  He humiliates and berates them,
manipulating them into killing Banquo.  He even uses an analogy comparing them to dogs. 
In effect, to do as Macbeth wishes qualifies them as members of a respectable breed,
while not to do so equates them with curs.  Macbeth, on the wrong end of the humiliation
in Act 1, uses humiliation of others in Act 3.