It's not a virtue to speak ill of others in private.

私底下说别人坏话不是一种美德。

youdao

Never speak ill of men behind their backs.

别在背后说人坏话。

youdao

You should not speak ill of your friend.

你不应该说你朋友的坏话。

youdao

But, of course, one ought not speak ill of the recently departed.

FORBES: Christopher Hitchens, A Fighter To the End (1949 - 2011)

By the time she graduated from high school, at the age of twenty, she had scarcely ever heard anyone speak ill of Scientology.

NEWYORKER: The Apostate

Bob Chaundy, former obituaries editor at BBC News, says there used to be a convention in obits, particularly in print, that you didn't speak ill of the dead, but that went by the wayside by the 1980s.

BBC: Margaret Thatcher and the taboo of speaking ill of the dead