If you intervened, you would very likely be charged with something and probably beaten as well. A friend of mine was playing with an airsoft gun when undercovers came out of nowhere (literally) and tackled him to the ground after slamming him into his car. My other friend told the officers politely that it was not a real gun and to please stop and he was punched in the face and charged with interfering in a police investigation.

Its cool to want to play gallant hero, but not prudent.