Why is it in history we look at Japanese imperialism as bad, but not western countries?
Who is the "we" that you are talking about?
My understanding is that colonialism/imperialism is widely recognized by historians as a capital-B capital-T, Bad Thing.
To be sure, there are occasional figures like Niall Ferguson who argue that western imperialism was "a force for good in the world".
But, I'd point to the recent responses to the Third World Quarterly controversy as evidence that academia will condemn anyone who presents a narrative justifying western imperialism while ignoring the well-known costs.