Bad rec is a crapshoot if the recommender doesn't tell the applicant "look, I can write you a letter, but I don't know
that I'd be able to write you the best letter". That kind of transparency should be the norm; if it isn't, then that is a bit of a crapshoot. I've written lots of letters over the years for admissions and scholarship recommendations. I've only had a few that I declined using the line I gave above. I'd never write a letter that I had to lie to write; I'd also never purposefully want to hurt a kid's chances.