Help Recovering S5 from downgrading?

[AC Question]

I've tried to downgrade my S5 from AT&T to Kitkat 4.4.2 with Odin 3.10.7, but the operation failed for some reason. Now I'm stuck with a message saying there was an error upgrading the firmware and I need to fix it with Kies. Kies apparently doesn't accept 'rooted' devices, so I'm stuck with Odin, which bricked my device in the first place.
Can somebody please help? I've been looking up solutions for the past 6 hours and none of them have had any effect.
I tried to flash stock firmwares and samsung recovery files a few times but it only made it switch between the yellow exclamation mark and bootlooping.
The Odin log always ends with:
<ID:0/005> Added!!
<OSM> Enter CS for MD5..
<OSM> Binary Check MD5.. Do not unplug the cable..
<OSM> Please wait..
<OSM> Checking MD5 finished Sucessfully..
<OSM> Leave CS..
<ID:0/005> Odin engine v(ID:3.1100)..
<ID:0/005> File analysis..
<ID:0/005> SetupConnection..
<ID:0/005> Initialzation..
<ID:0/005> Get PIT for mapping..
<ID:0/005> Firmware update start..
<ID:0/005> SingleDownload.
<ID:0/005> boot.img
<ID:0/005> NAND Write Start!!
<ID:0/005> recovery.img
<ID:0/005> system.img.ext4
<ID:0/005> FAIL! (Auth)
<ID:0/005>
<ID:0/005> Complete(Write) operation failed.
<OSM> All threads completed. (succeed 0 / failed 1)

It ends like this whenever I try to flash the stock firmware. It ends with 'NAND Write Start!!' whenever I try to flash the samsung recovery file.

Please help. I just want it to be back to the way it was: working. I don't care if it will go into Lollipop or Kitkat.