How would a hero justify blowing up a Federation building in London? Then assassinating a gathering of Starfleet Command? Heroes generally take on the bad guys, while doing their best to protect the innocent.
His actions make a bit more sense if he just thinks that Starfleet Command's upper ranks are stupid and/or corrupt. Getting them all into one place and killing them all would be faster than letting military courts handle it (especially if you think they're stupid/corrupt too).