‘Under the altar’ means at the foot of the altar, very near to Christ, near to the risen Lord in heaven. It is ‘the souls of them that were slain’ – without bodies; with bodies left behind in the world, in the grave.
The Bible is pessimistic about the future of the world. War, violence, inequality, and death will always be present, not improvement, not elimination of all man’s problems and the creation of a paradise on earth. We can expect only cycles of atheism and each grows worse up to the end. Death will come by violence, pestilence, famine, as well as the natural ways in which people are constantly taken from the earth.
Notice that they hold responsible all who dwell upon earth collectively. This is one of the Bible’s great generalisations which is perfectly legitimate. This statement obviously excludes believers, for they are not of the world, but it does not exclude anyone else. Are such generalisations fair? Yes, because the same spirit that caused some to put the martyrs to death, caused others to oppose God as far as they could. All are the enemies of God in their hearts and will not be reconciled to him.