Imagine the scene. The Wild West. Tumbleweed blows down the main street. Our hero faces the baddies, all six of them. Eyes narrow, hands twitch nervously over holsters. Someone goes for their gun and bang! bang! bang! The dust settles, and our hero picks up his hat which one now-dying baddie so inconsiderately shot off.
But who drew first?
The answer is ALWAYS one of the baddies. It's an absolute rule for heroes that they never start a fight. (Which of course is why the first Iraq war was OK as Saddam had invaded Kuwait, but the second one wasn't.) Your hero may suspect that the gang are about to mug him, but he mustn't whack them over the head until one of them has made a move. Dirty Harry may have got his name for his careless regard for the law, but he always obeyed the hero rules by inviting punks to make his day before he shot them with his Magnum 44.
It goes straight back to the nursery and 'who started it?' Whatever your reasons, you were in trouble if it was you. There's an interesting exception. Remember the first Indiana Jones film and that scene when a scimitar v horsewhip fight looms? Indy realises he doesn't have the time and shoots the swordsman. We laugh because our hero expectations are confounded, but deep down we're actually shocked by this breakdown of the hero rules - I can remember people talking about the scene when the film first came out.
In most cases I'd be saying make your characters active, not re-active, but this is one instance when they mustn't start a fight. Let the bad guys start things if you want the hero to retain reader sympathy.