I'm experiencing the same problem, and still working on solving it. But for those who are stuck in the boot screen and need to shut off their phone so that they can boot up into recovery (power + vol down), hold down BOTH volume buttons and the power button and the phone will shut off in a few seconds. I couldn't find this anywhere and was getting annoyed waiting for the juice to run out so I tried a few things and this worked.
edit: turns out it's just volume UP and power to 'battery pull' the phone, not both, although that will work because you're pushing on up
[ eeshu stylez ]
HTC One M8 Gunmetal