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:
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.
The murderers' attitude towards killing Banquo and Fleance are much different from Macbeth's attitude towards killing King Duncan. When Macbeth calls the murderers to his chamber to explain his plans, he reminds the murderers that Banquo has done them several injustices and that he has made them lesser men by doing so. Macbeth uses these words to incite anger in the murderers so that they will accept the task. The murderers do become angry and want Banquo to pay for what he has done to them. Macbeth, on the other hand, had no reason to want revenge against King Duncan. Before he is killed, King Duncan has highly and publicly praised Macbeth for his valor on the battlefield, and he has awarded Macbeth with a new title, Thane of Cawdor. King Duncan has not wronged Macbeth in any way, so when Lady Macbeth suggests that they kill him, Macbeth hesistates. Even when he does kill King Duncan, he does not do it with a sense of hatred towards the king--he does it with a greedy desire for the position that it will bring.