At the beginning of the scene and throughout his initial speech with the lord, Lennox uses verbal irony, which means he is saying the opposite of what he really means. For this reason, he adopts a sarcastic tone. The lord obviously understands exactly what Lennox is implying for, at this point in the drama, everyone is aware of Macbeth's malice.
He starts off by mentioning that Macbeth had pity for the kindly Duncan but that Duncan was dead, which suggests that Macbeth's pity had no value and implies that such pity was hypocritical. He then states that Banquo put himself at risk by wandering out at night and that it could be suggested that Fleance, Banquo's son, had killed his father, for Fleance fled after the murder. Lennox ties Fleance's actions to those of Malcolm and Donalbain after their father's assassination. They also fled, so surely they must have killed Duncan—their flight proves this. He ambiguously states that 'men must not walk too late,' suggesting that Banquo and others should have suspected Macbeth's malice from the outset and should have left Scotland before he could harm them, as he and others, such as Macduff (whom he refers to later), had done.
Lennox ironically emphasizes how terrible a deed it was for Malcolm and Donalbain to have killed their caring father. His exclamation that it is a 'damned fact' emphasizes just the opposite. It was assumed to be a fact but was, indeed, not so. He then sarcastically mentions how supposedly aggrieved Macbeth had been by this pernicious deed that 'in pious rage' he tore apart Duncan's two drunken chamberlains for murdering their king. 'Pious' is ambiguous since it firstly suggests genuine outrage and, secondly, hypocrisy. Obviously, Lennox means the second interpretation. He continues the sarcasm by rhetorically asking if Macbeth's retaliation was not noble, suggesting that it was actually the complete opposite—it was a heinous crime.
Lennox suggests that Macbeth had been wise in executing the two chamberlains, for anyone who would have heard them deny their guilt would have been angry. The suggestion here is that the anger would have been directed at them since the circumstantial evidence undeniably proved their guilt, and that knowing someone else and not they had committed the crime would have evoked just as much rage. Lennox states that, overall, it seems as if Macbeth has managed well for himself.
He continues by saying that if Macbeth had had Duncan's sons and Fleance imprisoned, they would surely have also felt the brunt of his vengeance for having foully murdered their parents. The remark is also bitterly ironic, for Lennox is suggesting that Macbeth would have executed the three just as he had executed Duncan's guards, apparently out of moral outrage. The real reason would have been to remove them permanently, for all three would have put him at risk, which they obviously eventually do by rising up against him.
In Act III, Scene 6, Lennox does not believe that the sons of Duncan and Banquo have killed their fathers. He speaks with irony to another lord that Macbeth "nobly" killed the "two delinquents" because anyone would have been "angered" to hear the men deny that they had slain Duncan,
...Did he not straight,
In pious rage, the two delinquents tear,
That were the slaves of drink and thralls of sleep?
Was not that nobly done? Ay, and wisely too,
For ’twould have anger'd any heart alive
To hear the men deny't. (3.6.11-16)
In this passage "the slaves of drink and thralls of sleep" the word slaves is a metaphor for the two servants who were drunken and sleeping and thus were exploited by any one else who was present as they guarded their king. Within this metaphor, too, is the literary device of personification, which is the attribution of human qualities to inanimate things. "Drink" and "sleep" are conditions that are personified, for only humans can own slaves.