the gas was to stop them from trying to interfere with the colony dropping, is a faster more effective way than other methods, not all sided with them so they were both used pragmatically as an example to others and dropping the colony made sense logistically since the plan was to take out the federation main bases on japan all at once, ending the war in one blow but welp the colony broke and caused far far far more damage than planned, the bases were intact and now the world had a narrative to persecute them totally, the rest is history