[One (M8) Q&A, Help & Troubleshooting] redpoint73:
The good news, is that your can't really brick this device while s-on (which it is), especially with the stuff you've done. It's simply stuck in a state that is not usable to you, that is very likely recoverable.

First off, I highly recommend you ditch the toolkit. This is for a couple reasons: one reason being, the toolkit only acts as a middle man for something you can (and should) know how to do yourself: simple one-line fastboot commands. And as you've seen, the toolkit makes it a little too easy to do something on accident, such as lock the bootloader. Which not only will typically render the phone unable to boot to OS (I'm surprised yours still did at first) as well as wipes all personal data on the phone. This would never happen, if you were doing the fastboot commands by yourself.

Another big reason to avoid the toolkit, it stopped being supported a long time ago, in 2014. Well before HTC stopped supporting this device (2016?). Since the toolkit is so old, it contains outdated versions of TWRP and SuperSU, which will cause a lot of problems for most folks. This includes you, and most likely the reason TWRP (outdated version) did no work for you.

My recommendation:

1) Your firmware is outdated (Lollipop). Since the bootloader is presently locked (relocked, basically the same thing) it is a good time to flash official firmware/RUU and bring your firmware up to Marshmallow. In order to figure out what firmware or RUU is right for your M8 version, we need more info. Do fastboot getvar all, and post the results (delete IMEI and serial number). If you don't know how to use fastboot, it's time to learn. Use xda seach function, and/or Google (although being a bit careful when following random websites outside xda) and learn the basics of fastboot. If you have specific questions, come back and ask. I'll be happy to help with those. But don't intend to give you a step-by-step instructions, for info that is posted in countless places already.

2) Once firmware is updated, unlock the bootloader again. You should be able to use the same unlock bin code as you originally did, to unlock the bootloader. Otherwise, just go through the process at HTCDev.com and get another unlock bin code. Flash the code with fastboot.

3) Once the bootloader is unlocked, flash latest TWRP 3.2.3-1 using fastboot. You can get TWRP from here: https://dl.twrp.me/m8/

4) Make a backup in TWRP of your stock ROM. This way, if you run into any problems rooting (or otherwise, want to reverse the process), you can still revert to stock. To backup the stock ROM, select BACKUP in TWRP, and select system and boot.img to backup. This is a very important step, that many folks (and guides) skip. But to me, it is not only best practice, but just common sense to make a backup and give yourself an "escape plan". I simply

never

root or make other mods to these devices, without having a backup of the stock ROM.

5) In TWRP, flash current version SuperSU 2.82 from here: https://xdaforums.com/apps/supersu/stable-2016-09-01supersu-v2-78-release-t3452703

But to be honest, I would suggest Magisk at this point. It is a newer root method than SuperSU. And SuperSU appears to be dead, anyways. Not only is Magisk still being updated and supported, but it seems to work a little more reliably. Plus, Magisk has a ton of cool and useful features, that SuperSU does not. Suggested Magisk version is 18.0: https://xdaforums.com/apps/magisk/official-magisk-v7-universal-systemless-t3473445

6) After flashing your root file of choice (SuperSU vs. Magisk), reboot and you should have a usable (and rooted) operating sy system.