No there wasn't. And that's why I don't like Zinyak. He had a good voice and good dialogue, but blowing up Earth for no reason just made him an ineffectual troll villain to me.
And no it wasn't to break thw Boss' spirit. Zinyak himself calls the Boss a sociopath. A sociopath isn't going to care about the lives lost, only that you wrecked their stuff. So Zinyak did something that could only ever piss the Boss off.
Of course Zinyak and logic don't seem to get along at all. Take Johnny Gat. He's more of a threat than the Boss how? Memetic badass or no, he's also a follower and has needed his butt saving more than once.
Yeah yeah I know it's comedy. Well to me, comedy is pointless without at least one straight man, and in SR4 everybody's a clown.