Lennox is the Fickle Thane: he gravitates to whoever is in power. First, he's "loyal" to Duncan; then Macbeth; finally, Malcolm.
Lennox is none too bright. Shakespeare uses him as a mouthpiece for dunces. He has many dim-witted, deadpan lines that show verbal irony. Like this one in Act II, just before Duncan's murder is discovered:
Monty Python has made many skits out of guys like Lennox and lines like these.
Then, there's Lennox who echoes Macbeth's lies. The propaganda pitch man. Look at these gullibly ironic lines from after the murder:
And then we return to the one-liners. At the banquet scene, when Banquo's bloody ghost has already taken the spot:
And there's the Lennox that's both so gullible and fawning for power that he has to get the last line in after Lady Macbeth has rousingly dismissed the guests:
Macbeth needs a doctor about as much as the Bleeding Captain needs a Band-Aid.
And now, my favorite. A monologue that's so bad that most directors cut it from the staged play. Did Shakespeare really write this scene? It's so full of logical fallacy that it's parody:
Here's a translation: "Duncan was murdered, and Malcolm fled. Therefore, he must have murdered his father. So, if Banquo was murdered, and Fleance fled, it must mean--Ipso facto--that he murdered his father too."
Sounds again like Monty Python. "If she weighs the same as a duck...she must be made of wood...and therefore...a witch! Burn her!"
Lennox is being craftily ironic. He's no dunce. That's a complete misread.