A few big good/bad decisions are more important than thousands of trivial decisions.

Balancing good against bad is not clear, I agree, but that makes it all the more important to think about it when making a big decision such as what job to take.

Its hard and people do not even agree about how to balance it - e.g. whether the ends justify the means.

Well that's my point. Someone working as a geologist for the oil industry can think climate change isn't real or not a problem so they can view their job of finding more oil as being a clear benefit for society.

No amount of trying to tell them "But you are doing something bad by helping the oil industry" will make them reconsider if they are good or bad. But I also wouldn't say they are necessarily bad people. You can also simply just be wrong or misinformed when you make your moral calculation.

Good people do bad things sometimes. Bad people do good things sometimes. That's why the framing of badness as a taint isn't really helpful IMO. It creates a "us vs them" sort of attitude that ultimately doesn't address whatever issue is at hand.