Rooting an LG is inverted and Windows only unless you go to Kingroot hell.
Rooting the M8 follows the standard Android model, not surprising because HTC built the first Nexus and the first Android.
1. Unlock bootloader (you have to get the token from HTC but it's trivial)
2. Install TWRP using fastboot
3. Make a nandroid backup of your stock setup
4. Either flash SuperSU or install a rooted rom of your choice from TWRP
And you can do that from Windows, OS X, or Linux.
If you want, you can spend $25 and get Sunshine s-off. With that you can manage firmware independently of the rom. A big advantage of that is being able to upgrade the audio DSP to 24-bit 192 kHz and depending on the mod you go with, flash the HK audio libraries (originally available only on the Sprint Harman Kardon M8) or the Dolby libraries (now stock on the M9).
Boomsound is quite good, the HK mod is quite better.
The M8 was one of the last GPE phones. If you prefer standard Android over HTC Sense, you can flash a GPE rom. And the rom collection overall is quite good, including a ports from the M9 so you'll be ahead of the update game if that's what you want.