He knows his secret love for Rodney could destroy his career and risk both of them, so he tries to make Rodney evil and single-minded and uncaring, but his own drive towards potential self-destruction manifests itself as a more physical destruction of the biggest thing he can think of (and no, that awesome turkey sandwich doesn't count) and thus Rodney blows up the majority of a solar system [insert comment about how its lack of total destruction reflects John's hidden hope of survival].