The prisoner is shot because the act of shooting a prisoner is near instantaneous. The action has no duration that would justify the use of the present participle, unless it were an order that had been given and were still to be carried out. Bombing raids over the third reich could go on for hours at a time and be carried on over consecutive nights, so the factories are being bombed. Think of it like this, the decision to shoot the prisoner could be taken, the order given and carried out - and even the mess probably cleaned up - but still during all this while the factories could quite feasibly have been being bombed, continuously, and without respite. Eventually the factories would either cease to exist altogether, or the bombing would cease and the shell-like structures would have simply been bombed.
When making a heading for your essay, it should be located in the beginning of the whole essay. This is what makes the essay come to life and what start's off the essay well.
