I already have a plan for this situation
1. Find a sharp object (knife, plastic shard etc)
2. Take off shoes to quieten footsteps.
3. Sneak up behind armed attacker.
4. Insert sharp object into the side of their neck and push it forward out the front.
5. Crisis averted (unless there are more terrorists/gunmen or the first guy saw you coming).
Seriously though, when I think about the huge number of people Brevik murdered on that island, I cant help but wonder how many opportunities people had to lower that count. I mean, he killed 69 people! Here in Australia, our own psycho (Martin Bryant) killed 35. Another 38 killed by the terrorist on the Tunisian beach and 32 killed by the crazy South Korean at Virginia Tech. Its a terrible and astounding thing, to realize how much damage one person can do with guns. I also wonder how many lives were saved, when those guys brought down that terrorist on that Euro train recently.