Recover data on internal memory of a dead HTC One M8
Double-trouble. If the device cannot even be powered up, you can't access its internal storage to rescue your data. As soon as the software is flashed and the device reset, that data is gone – and without root powers, rescuing data from internal storage is close to impossible (so at least your WhatsApp data would be gone for good). There are very few tools which might help you on a non-rooted device (e.g. DiskDigger), but I wouldn't count on that.