One panelist on the "jumped the dragon" panel said that the Xanth series jumped the dragon after the last book in which someone died; thereafter, Anthony seemed too fond of his characters to actually kill anyone. He had a point. It's hard to convince people that there are real risks in the world when, really, there aren't.
It still hurts to knock them off.
Sometimes you can get away with knocking off the villains of the piece. One advantage of humanizing the villain is that his death is convincing evidence that people really do die here. Innocent bystanders can be more tricky. It's easier to kill them off if they are anonymous, but it's harder to make the reader care, or convince them that the main characters are in real danger. I still remember a comic book sequence in which a villain wandered the universe, looking for the heroes he hated, and slaughtering large batches of people. It was after a fight with known characters, when he kicked one of them while he was down, petulantly saying it was all his fault for not taking his deal (he had offered to allow him to kill some of the heroes, too, not realizing he had reformed) that he came across as evil. It takes a skilled hand to humanize bit parts enough to make the reader sympathize.with them enough for it to bite.
And, of course, the degree of danger that the main characters are in will do a great deal to determine the tone. A story where the reader really feels that anyone could die at any moment has a much nastier tone. . . .