Macbeth's strange behavior is explained away by Lady Macbeth,"...My lord is often thus,/And hath been from his youth."
Since the murder of King Duncan, strange things have been happening. In Act II, scene 4, a seventy year old man tells Ross that never in his long life has he seen so many unnatural happenings like an owl killing a falcon, heavy darkness when it should be day, and the unnatural behavior of Duncan's horses. "Tis said they ate each other."
Since it seems to be a time of odd behavior, Macbeth's odd behavior seems to be just part of a litany of strange happenings. Lady Macbeth's explanation fits right into the current pattern.
Another thing to remember when reading the scene is who is Macbeth talking to and just how much of it is meant to be heard by the gathered thanes. For example is he talking to them when he talks about blood being shed? He could be "talking to himself" as it were and Lady Macbeth brings him back to the group and the toast.
These are all men who have been to war and perhaps have demons of their own that haunt them. Kind of like Scottish battle fatigue.
Do these thanes begin to suspect things aren't what appear to be? Probably, since this as a time full of fear and suspicion. With constant warfare, alliances often shifted. The Thane of Cawdor's defection is a good example of this. Who do you trust?
The easiest way to find out what a character thinks about what happened in a scene (if that character participates in the scene, but doesn't have many lines) is to see what that character says the very next time they are on stage.
Lennox, who attends the banquet and leaves wishing good health to Macbeth, shows up just two scenes later in III, vi, talking to another Lord. This scene is short and the other character simply named "Lord," so we can assume that Shakespeare included the scene to let us know Lennox's reaction to Macbeth's behaviour at the banquet.
While Lennox lists all the explanations given for the deaths of Duncan and Banquo, it is clear in this scene that he now fully suspects Macbeth to be behind the murders. He ends the scene advising stealth and backdoor maneuvering, hoping that Macduff might save Scotland, and planning to alert those in England (Duncan's son and others) of the need for potentially rising up against Macbeth and his blood-stained rule.
I suspect that the thanes around the table feel first confused, then possibly suspicious of Macbeth's outburst and Lady Macbeth's dismissal of them. It has already been acknowledged that Banquo is missing, and the thanes cannot understand why such a loyal person as Banquo would be absent from the dinner. Macbeth's outburst comes just at this time of speculation, indirectly suggesting that Macbeth has something to do with Banquo's absence. When Lady Macbeth dismisses them from the banquet hall, it is likely that they start to wonder about Macbeth's capacity to fulfill the role of king as Duncan had. After this, Macbeth begins to increasingly lose supporters as suspicions fly and others wish to overthrow his tyrrany.
Macbeth's repeated outbursts on seeing Banquo's ghost sitting at the banquet table and shaking his 'gory locks' at him arouse great confusion and suspicion in the minds of the thanes. Since the ghost is hallucinatory in nature and visible only to Macbeth, neither Lady Macbeth nor the lords like Ross and Lennox can understand the weird behaviour of the king. Macbeth's sense of fear and guilt give birth to the ghost and he addresses the creature in such ways and manners that the lords suspect the new king. Lady Macbeth tries to divert the attention of the guests and reprimands her husband without realising the source of her severe agony and tension. The banquet is never really begun, and when Lady Macbeth summarily dismisses the guests, the thanes of Scotland must have confirmed Macbeth's complicity in the crime.
They are no doubt thinking that their King is losing it. He is talking to the air in front of them. They may be thinking that he is merely drunk at best or totally insane at worst. In any instance since it is so apparent to Lady Macbeth that her husband is screwing up since she is trying to cover it up, it is just natural to assume that the end is coming. This is a wonderful tool used by Shakespeare to show us just how messed up Macbeth is becoming. Such a dramatic and interesting way to show the decline of a king!