Macbeth did have some feelings for Duncan. He says he is there in "double trust" because Macbeth is his loyal subject and kinsman. Macbeth also has some feelings for Banquo, as they seem to be friends. Macbeth kills both of them, regardless of his feelings for them, so that he can gain the throne.
Macbeth approaches these two killings in different ways. He has to be goaded into killing King Duncan by his wife, but he takes on the job of killing Banquo on his own. He suffers a lot of guilt over both murders.
Very stark contrast in the killings, however, as his murder of Duncan was an offensive strike to gain the crown, and his murder of Banquo was a defensive tactic in order to try and keep the crown. He also certainly didn't seem to have the same level of internal conflict in deciding to kill his friend Banquo that he did in killing Duncan.
The two killings are alike in that he did not follow through with the killings alone. His wife helped him with Duncan, and he hired killers to kill Banquo.
What seems different to me is the drive that lead him to both killings. He did not seem to see a future need to kill anymore when he planned Duncan's murder. Kill him, and be king. But once he plotted Banquo and Fleance's murders, he was looking down the road at any possible adversaries he would need to stop.
Compare Macbeth's Killing of Duncan and Banquo
compare macbeth's reasons for killing duncan with those for killing banquo
There are some significant differences. In the killing of Duncan, Lady Macbeth is at least half as responsible as Macbeth himself is for the crime. She urges his hand, he plunges the dagger. Both Macbeth and Lady Macbeth are propelled by their greed.
But in the case of Banquo, Macbeth alone decides that this potential rival must die. And this time, he refuses to carry out the murder himself. Instead, he sends hired assassins to kill both Banquo and his son, Fleance. Fleance escapes, but his father dies. Macbeth has shown himself in both cases to be weak, susperstitious, and easily led. He is deadly too, but still a coward. He lets his wife have the upper hand (much a no-no in Shakespeare's day), then his cowardice and callousness is evident when he murders Banquo and tries to kill his innocent son all so that he may have a "smooth path" to the throne.
The reasons for which he killed the two men are very different. The reason why he killed the then King of Scotland, Duncan was so that he could have the path of his becoming King himself cleared. Had Duncan not been murdered and his sons, including his named heir Malcolm, had not fleed out of fear of their own safety, Macbeth would have not been crowned. There was no personal reason. Duncan had been a good king, judicious, virtuous, noble, just, honest, kind, generous and wise. He had never given him or anyone else a reason, for which to be murdered, He had indeed, lived a life of absolute piety and deserved respect from all.
The reason why he killed his good friend and coomander-in-chief of the army, Banquo, was because if there was anyone who MIGHT have had a suspicious regarding his involvement in Duncan's murder was Banquo himself. A lso Banquo had een present when Macbeth's future as king had been prophecized by the Three Witches. As such, Banquo's suspicion could have proved disastrous to Macbeth's kingship, which is why he decided to appoint murderers to kill him.