It is usually difficult to hard brick a device but a soft brick can be fairly easy to accomplish. During the early days of development I was testing an experimental build for the G2 when it refused to boot. Thinking I could get it to boot with a different kernel, while being stuck in fastboot mode wasn’t the best idea. If you are stuck in fastboot, I recommend you reflash a custom recovery and continue on your way. If a kernel is improperly flashed the phone will present you with a screen that says LG Security Error. Upon reaching this screen, the only option to return to a fully functional phone is to restore to stock.
Follow these instructions on XDA written by hyelton and you should be up and running.
LG G2 Stock Firmware (Go Back to Stock)
After following these steps and restoring to stock software, you can now root and flash a custom recovery as normal.
Best of luck!
The Lost Digit 