The claiming of planets is one thing. There was basically a war going on and the rabbits had to make sure Devan didn't get so powerful that they couldn't stop him if Jazz failed. As for killing everything in sight, based on my understanding of Jazz' character, I'm pretty sure that's just how Jazz operates. He certainly seems the "shoot first, ask questions later" type. Besides, If he'd been ordered by the Galactic Rabbits to kill everything, there'd be nothing left alive and nobody to conquer.

Ah, but did Devan actually build his time machine or did he just give his goons the plans and have them build it for him? If it's the former, I concede the point, as Devan is one of the greatest geniuses in the known universe. But it seems plain that many of his soldiers rival the Dopefish and the Ravenous Bugblatter Beast of Traal in stupidity. Since Devan had to coordinate his troops to beat the Jackrabbits to wherever they were going, it may very well be he had his troops do his dirty work. If Devan's soldiers messed up in the construction, who knows what would have happened.

And Devan can't die. He's the arch-nemesis of a video game hero. They practically never die.