a change for the worse; the act of making something worse (opposite of improvement).
ソフトの最新版は多くのユーザーから改悪だと非難された。
The latest software update was criticized by many users as a change for the worse.
税制改正が一部の負担を増やし、改悪だという意見が出た。
The tax reform increased some people's burdens, and opinions arose that it was a deterioration.
設計を見直して改悪を避ける必要がある。
We need to revise the design to avoid making things worse.