A Worn Soldier and Comedic Relief

"O heavens, can you hear a good man groan
And not relent, or not compassion him?
Marcus, attend him in his ecstasy,
That hath more scars of sorrow in his heart
Than foremen's marks upon his battered shield,
But yet so just that he will not revenge.
Revenge the heavens for old Andronicus!" (Act IV, i, 123-129)

I know Marcus is sympathizing with Titus and what has happened to Lavinia and his sons, but I am confused as to what he means when he says "But yet so just that he will not revenge." Why would he not want his brother to avenge the vicious acts that have occurred? Does he think that Titus will be no match for Saturninus because of his old age and years worn as a soldier of Rome?

"And pray the Roman gods confound you both" (Act IV, ii, 6).
"That you are both deciphered, that's the news" (Act IV, ii, 8).
"And so I leave you both-[Aside] like bloody villains" (Act IV, ii, 16).

I found Young Lucius to be very comical in this scene. I don't know if it was meant to be, but the image of a little boy making sarcastic comments about Chiron and Demetrius while they're standing right there, seems to serve as some sort of comedic relief for what has happened and what is about to happen.