In Shakespeare's Macbeth, Act 3.6, Lennox may be being politically careful in his speech. He says nothing that could cause him to end up like the murdered people he mentions--Duncan and Banquo--or the person he suggests is now in danger--Macduff--due to his absense from the "tyrant's" feast.
The key to understanding Lennox's true feelings in the speech is "tyrant." Lennox considers Macbeth a tyrant. It is hard to hear that and then still conclude that Lennox really believes Malcolm and Donaldbain killed their father, and, even more unlikely, that Fleance killed Banquo.
It is much more likely that Lennox is using sarcasm, if the speech is performed with a sarcastic tone of voice, or at least irony, if the tone of voice is not sarcastic.
Shakespeare often wrote ambiguously, leaving the words of a character to be interpreted by directors and actors. This is probably one of those instances. One thing is relatively certain, though: Lennox is questioning the theory that the deaths he mentions were the result of sons killing their fathers.
In Lennox's speech of Act III, Scene 6, there are clearly examples of verbal irony and, as readers know, irony is more effective at times in communicating an outrage or incongruity than is simple statement. In the opening lines of Lennox,
My former speeches have but hit your thoughts/Which can interpret farther. Only I say/Things have been strangely borne (III,vi,1-3)
he suggests that there is a difference between their perception and his. With irony, he suggests how great this contrast is. First of all,
The gracious Duncan/Was pitied of Macbeth: marry, he was dead. (III,vi,3-4)
That is, Duncan was so lamented by Macbeth, but he was dead already. And, Banquo was out too late; "Men must not walk too late" (III,vi,7). Here, with irony, Lennox implies that Banquo was at fault for his own death, but really suggests again that Macbeth is the villain. Then, the verbal irony becomes clearer:
How it did grieve Macbeth! Did he not straight,/In pious rage, the two delinquents tear,/That were the slaves of drink and thralls of sleep?/Was that not nobly done? Ay, and wisely too;/For 'twould have angered any heart alive/To hear the men deny 't....(III,vi,11-16)
With such biting irony, the listeners cannot help understanding the incongruity of two drunken men having killed Banquo. The only reason for their murder is the fact that they would have denied having done the act, an action that would point the finger at someone else; namely, Macbeth himself.
If you look at the whole speech that Lennox makes to that one lord, it is totally sarcastic. That is what he is saying that I see as criticism of Macbeth. There is no one thing he says that literally says anything bad about Macbeth, but all the good things he's saying are sarcastic.
He says it was lucky Macbeth killed the servants so they couldn't make people mad by denying their crime. He says that Macbeth was really so sad when Duncan died -- it's all sarcastic and meant to say that the opposite is true.
He can't come out and say this stuff because Macbeth is still king and could have him killed.
When Lennox first appears in Act 2, Scene 3, he is obviously a very young, naive lad who is ill at ease and unsure of himself. When Macduff goes into Duncan's chamber leaving Lennox alone with the grim and silent Macbeth, the young man feels obliged to talk about something, so he talks about the weather.
The night has been unruly.Where we lay,
Our chimneys were blown down, and, as they say,
Lamentings heard i’ the air, strange screams of death,
And prophesying with accents terrible
Of dire combustion and confused events
New hatch'd to the woeful time. The obscure bird
Clamor'd the livelong night.Some say the earth
Was feverous and did shake.
Then Shakespeare uses this character again in Act 3, Scene 6, to convey an impression of how much has happened to Scotland since Macbeth's coronation. There has been a striking change in young Lennox which can only be attributed to the traumatic events he has experienced and witnessed in a few short years. Now he seems grown up, sophisticated, worldly wise. He has learned to be guarded in his speech and not to babble everything he knows. Macbeth has secret agents everywhere. In Act 3, Scene 3, Macbeth tells his wife:
There's not a one of them but in his house
I keep a servant fee'd.
Lennox has learned innuendo, imputation, and discretion. His entire speech to this unnamed Lord is in strong contrast to the way he thought and spoke in Act 2, Scene 3.
Shakespeare's purpose is to show how much things have changed in Scotland since Macbeth took the throne. In Act 4, Scene 3, Ross tells Malcolm and Macduff about conditions at home.
Alas, poor country,
Almost afraid to know itself! It cannot
Be call'd our mother, but our grave. Where nothing,
But who knows nothing, is once seen to smile;
Where sighs and groans and shrieks that rend the air,
Are made, not mark'd; where violent sorrow seems
A modern ecstasy. The dead man's knell
Is there scarce ask'd for who, and good men's lives
Expire before the flowers in their caps,
Dying or ere they sicken.
Everybody now feels sure it was Macbeth who murdered Duncan and intended to murder Malcolm and Donalbain. Everybody knows it was Macbeth who had Banquo murdered and tried to have Fleance murdered at the same time. Everybody hates Macbeth for his criminal acts--and this hatred, bordering on outright rebellion, has forced Macbeth to become a tyrant who rules by fear. All of this is shown through the formerly callow young Lennox, who not only imparts a great deal of information but represents the bitterness and disillusionment of all the thanes and most of the Scottish people. This is Shakespeare's way of encapsulating time, atrocities, dissatisfaction, rumors, and countless incidental events into one single character's monologue.
I am playing lennox in a production of macbeth and in my mind the character is cautious in his words so as not to end up like banquo because macbeth will get rid of him if he realises lennox knows this truth. However at the same time lennox wants to share his thoughts with another lord who, I believe, he trusts so uses sarcasm and irony to get his point across