I'm on AT&T and had the LED notification issue. AT&T had no clue what the problem was. Found the solution, at least for me - I think it's a bug with the HTC phone setup. If you enter/login with a Google account DURING THE PHONE SETUP PROCESS, it CAN disable the LED settings. The trick for me was to factory reset the phone (I know this SUCKS), proceed with the phone setup, get to the home screen, and ONLY THEN link your Google account.
The caveat - I think this may only be an issue with certain stock of HTC One M8's. I also think it depends on what phone you're coming from. Google backup saves your previous LED settings from you last phone, in my case a Galaxy SIII, and tries to transfer as many phone settings as possible to your new phone, to save you the hassle. Unfortunately, I think the LED settings for the Samsung phone screwed with the LED settings in the HTC. Whatever. Point is, this fixed the problem on THREE of the HTC One M8's I had.
I had three because the first one's volume down rocker stopped working after 32 days of owning the phone. So I swapped it for another phone through AT&T - the phone they replaced it with had a weird brown hue to the white areas of the screen. But I confirmed that it had the weird LED issue if you linked Google in the setup. (Apparently some One M8's are made in China and some in Taiwan, and the screen RGB hues are different for the two - great.) Finally, in the third phone I got, the screen was fine, and I averted the LED issue by linking Google after setup. HOWEVER.....this phone's volume rocker is so loose it rattles, like yours, whereas the other two did not.
**HITTING HEAD AGAINST WALL
(Lesson being, after being a Samsung user for the better part of 6 years, HTC's materials are way more impressive, but their consistency in construction is craptastic.)
Hope this helps some of you.