Japanese imperialism was an attempt to get out from under the boot of US imperialism. The US showed up, forced their nation open to western powers,and spent the next 70 years intimately engaged in all their important decisions.
Japanese conduct during the 30s was atrocious. They tried playing by the moral code in earlier wars, the US and other Western powers repeatedly reneged on deals and forced Japan to return any possessions.
I don't think that's quite fair. It's true that after the arrival of the US, Japan was apparently fed up and changed its approach. But does that make invading and subjugating their neighbours justifiable?
Sort of. If you feel there's a greater power out there that's building up to invade you and your neighbours, forced unity can seem like the lesser of two evils. The US was not super friendly in the first half of the century, Hawaii and the Philippines were both recent military conquest dangerously close to Japan.
Japanese conduct during the 30s was atrocious. They tried playing by the moral code in earlier wars, the US and other Western powers repeatedly reneged on deals and forced Japan to return any possessions.