You know Rossiu had to fall. He was such a dick that he had to fall eventually, and Simon had to end up back on top. Rossiu was willing to sacrifice everyone left on the planet to save the people who got the invitations - basically, the Gurren Brigade and their families. Simon had no choice in the matter - he got out of jail yet, had the Core Drill thrown at him, and basically said, "Let's go kick some ass."
Simon didn't risk anything, he did the only thing he could do at the time. The alternative was to stay on the planet and wait for the moon to crash.