It justifies doing bad things for "the greater good".

It is purely people justifying doing the evil things they personally desire by claiming some unverifiable benefit that accrues to hypothetical future people who may never even exist.

idk if it's all bad, I remember reading about it maybe a decade ago when rationalists talked about it. Like one good idea is donating to charities that are more efficient. But it seems like mostly BS wrapped up in mathematics to make it seem more rational. Some assumptions are not necessarily true but if you disagreed then you were immoral and irrational. It also comes with a form of very elitist virtue signalling like forming clubs where people bloviate about this.

Also a problem with charity, especially in the US, is as the IRS defines it, it's not necessarily charitable. So it's easy to confuse altruism with plain old politics, vanity, and for-profit business disguised as a tax-exempt and then argue that anyone that disagree is not rational.