Eraserhead is simply biased because of his own power, erasing. In front of him, nobody is special because he can erase their quirk. So, one person is the same as the next one. However, when Eraserhead isn't present, there could be a villain who could slaughter ten thousand people. If a hero sacrifices himself stopping that villain, the hero has saved ten thousand people and lost one (himself). If he does like Eraserhead suggests and doesn't sacrifice himself, he saves a single person, himself, and loses ten thousand people. Eraserhead simply can't see this as due to his personal power, the villain wouldn't be able to murder any more people than any random joe from the streets, so any cop with a gun could stop the powerless villain.
It's really troublesome to have a teacher who can't see beyond his own limited point of view.