Just in case anyone else is having trouble finding how to invoke the slow motion camera, instructions are here:
http ://www .htc.com/us/support/htc-one-m8/howto/464939. html
(take out the spaces, I don't have enough posts for posting a link)
Recording videos in slow motion
Relive high speed action bit by bit and add a touch of suspense to your videos.
Important: Audio is recorded when you're in this scene, but will only be available when video is played back in normal speed.
Open the Camera app.
On the Viewfinder screen, tap (the three vertical dots icon) to show the camera options.
Tap [AUTO}, and then tap (blurred figure looking icon).
Tap (video camera icon) to start recording.
To stop recording, tap (black square) .
Be sure to be in Video mode before trying these instructions.