You may not have said implicitly that death threats are justifiable but you seem to apologise for the behaviour that preceeded it.
Muslims were singled out.
How?
The images are a red herring, it’s the assumption that Muslims should just accept being treated differently that causes anger.
How were/are Muslims treated differently? If they are, is anger to be tolerated or submitted to in your opinion?
He seemed to lack the basic knowledge that Muslims don’t share any images of the prophets, even positive ones.
But he wasn't a Muslim himself. So it isn't something taboo for him to do. Again, people can be offended, that's not a criminal offence. Neither is it justification to committ a criminal offence. So why apologise for behaviour that is illegal?
I think part of the problem here is that people don't understand that offence is ok. If you choose to follow a particular religion, that's fine. But you cannot expect others who don't follow it to adhere to it too, just because you want them to. In a secular school, in a secular country you have every right to express your opinion, to discuss and debate to explain why you find something offensive. What you don't have the right to do is stop the debate with threats and bullying. This is where the fault lies in this instance, not with a teacher showing an image.