A handgun has almost as much killing power as a rifle. Most rifles people have are fire 5.56x45mm rounds. The penetrative power of those are reasonably low. Now, 7.62 will probably punch through any body armor they’re wearing, but that’s fairly uncommon I’m the US. With the NGSW we’ll see more larger rounds, but until then your handgun will kill about as well as their rifle, assuming your close enough which a self defence situation would imply.
A longer gun is better for longer ranges. If you’re at longer range, probably just get out of the way instead of thinking you’ll fight back. Most likely you’ll just make the situation more confusing and no one will be able to identify the “good guy with a gun” and you’ll get shot, by police or otherwise.