Some terrorists are ready to be suicide bombers. If he detonates a suicide bomb next to you, then your question is moot.
You have no chance to think whether you should or should not do anything.
If a few terrorists use assault rifles to kill, then Run Hide Tell is correct.
Although Tell is superfluous. A terror attack is usually in a metropolitan downtown busy area, chosen for maximum impact.
Once the attack starts, the whole city would know about it in these days of smartphones.
Run Hide is correct, as terrorists come prepared with bombs and firearms with lots of ammunition. They outgun you.
You have no firearm weapons. If you block his live rounds with your body, you may be acclaimed as a dead hero the next day.
But you will be giving the terrorist what he wants - maximum body count.
If the terrorists are unable to kill a lot of people, they get frustrated. They are looking for impact, to make a statement.
Usually terrorists do not plan to go away alive after the attack.
Let the swat teams handle the terrorists.