I've not been in that position, but in my head I'd be the hero and intervene, get all the adulation and applause but lets be realistic, what is the point?
First of all, my Mrs would absolutely kill me, she wouldn't think my actions were heroic. I've got 3 kids, 2 step kids and she would see it as i risked my life to intervene and prevent theft of insured goods. My mam would say the same she'd go wild. My mates would be torn between thinking it was stupid, and cool, i'd probably earn a nickname off it and i may get a nice gazette headline and a free tesco shop but i also may get stabbed, or shot, or filled in for my trouble and they'll still probably get away with whatever they want and they'd only go do it again elsewhere. It wouldn't prevent anything, the cons of stepping in far outweigh the pros.
So my head says yeah step in, save the day but then realistically i wouldn't, and i'd be hoping my **** collapsed and i just sit still and shut up to get out alive.