It's not the guns that bother me. Yes, it goes against the modern Batman ethos but he has been shown with guns before.
It's how he's using them. Look at it! Just standing there pumping lead into the walls. This isn't remotely careful, planned or controlled. It's brash, messy and will leave heaps of evidence. And that many bullets and he doesn't even hit anyone! Any other character and I wouldn't mind so much, but this is Batman.