Pretty much, if you kill someone, you're a hacker.
Around a decade ago, when I was playing America's Army, I got called a hacker literally daily. It's probably down to weekly now with Battlefield, but it's still a very common occurrence. Last night I was on a low populated Tehran rush server. I was in a tank getting locked on repeatedly by someone with a jav. I look around, see the guy behind me and kill him. Instantly, I'm a hacker because I'm able to look more than one direction.
You get used to it after a while, and just laugh at them. It doesn't help with the lousy hit detection the game has. I've killed people when they were around a corner even on my screen. Many people confuse this with hacking.