Galactus made carnage (kasady) his herald giving him the power cosmic?

If Carnage didn't turn on him immediately (and get destroyed), he'd turn on him later. Carnage LOVES killing. Whether it's the sea of gore he wades through after a massacre, or the agonizing intimacy of a serial killing, he absolutely loves it. He loves every aspect of it, from the shrieking struggle and fight, to that moment when it truly dawns on a person that they're about to die, to that moment when the light finally goes out of their eyes.

And with the Power Cosmic, the universe is his canvas. However, he'd have nearly unlimited power to do as he pleased. There'd be no challenge to killing. No effort. The quickest way to stop someones enjoyment of something is to make it really easy to do and make them do it en masse. After the third or fourth planet, he'd get bored and turn on Galactus, or otherwise quit being a Herald, just so that killing is fun again.