I will not speak of Caesar who once turned the plains of green bloodred with all those veins in which he plunged our steel.
Petrarch is not being coy by not talking about Caesar in detail here. He's using paralipsis to show us that Caesar had a reputation as a vicious dude. It's like Petrarch just… won't… even.
He puts this recollection of Julius Caesar in here to remind his audience that Italy has its own savagery in the past—and that sometimes it turns on its own people.
It seems—who knows by what malignant stars— that now the heavens hate us, and thanks to you, to whom much was trusted. Your disagreeing wills despoil the fairest part of all the world.
Karma appears to be at work here, though Petrarch seems befuddled by it. Do they deserve the current state of violence because of past actions of the Roman Empire? Do they deserve it because the nobility are so incredibly short-sighted? Petrarch cannot guess.
But he does know who to point the finger at for his sorrow: at those "to whom much was trusted"—in other words, the ones who Fortune favors (the nobility).
Not only have they been entrusted with the resources of the land and lots of coin, they've also been entrusted with the lives of their people and the health of the country.
And as Petrarch has said before (just check out stanza 3), they inherited the best of all countries.
What fault, what judgment, or what destiny makes you harass your wretched neighbor, and broken, scattered fortunes persecute, and seek out foreign friends, glad to know that they shed blood and sell their souls for money?
The structure of this entire section is based on erotema. By asking a well-developed and pointed rhetorical question, Petrarch emphasizes his own indignation and the sheer stupidity of the warring nobles' behavior.
To drive the point home, our poet catalogues the sins of the nobility. Of note, he can't believe that they would fight with their neighbors and worry everyday people with the inconveniences of war.
Also, are they seriously happy about buying their friends and then sending them off to die?
What kind of people are these?
I speak to tell the truth, not out of hate or scorn for anybody.
We have to remember that Petrarch is calling out the rich and powerful in this poem. It's a pretty risky thing to do—not very diplomatic. But Petrarch feels that it is his duty to speak out.
Still, it's probably best not to give a bad impression, like you're working for someone who has a grudge. So this couplet is well-placed backpedalling, right at the end of the harshest stanza of the poem so far.