it left my mother very sad and resentful.
It - 代表這語句之前發生了的事件. 假如是:
Our dog was killed by a traffic accident last night, it left my mother very sad & resenful.
left (p.t of leave) = 帶給, 留給, 留下, 有時可解做 導至
left (n) = 左, 左方
Cambridge dictionary :
If something leaves something else, a part or effect of it stays after it has gone or been used.
EXAMPLE: Far from improving things the new law has left many people worse off