Is it obligatory for a Muslim to repel injustice from a Muslim and a non-Muslim, even if he fears being subjected to the oppressor's tyranny, and is that considered casting oneself into destruction? What is his duty if he sees an oppressor forcefully removing a Muslim woman's hijab?

A Muslim is commanded to support his Muslim brothers and is required to support the oppressed, even if the oppressed is not Muslim. This falls under the category of enjoining good and forbidding evil. For the obligation of supporting the oppressed, ability and safety from considerable harm are prerequisites. This is a communal obligation (fard al-kifayah). Whoever is able to support the oppressed and does not fear certain or predominant harm, it is obligatory for him to do so, and he is sinful if he abandons it. Whoever does so despite fearing harm will be rewarded, as long as this support does not lead to greater harms.