How can I return to locked boorloader, unrooted and original ROM state on Asus Zenfone 2 Laser ZE500KL? I need to return the phone for reparation. The handset is broken. The warranty is void in the current state of the mobile phone. 

How do I return to its original state?
ASUS already upload stock ROM for ASUS Zenfone 2 Laser Series, You can download the ROM's via download links below :

Asus Zenfone 2 Laser ZE550KL:
http://dlcdnet.asus.com/pub/ASUS/ZenFone/ZE550KL/UL-Z00L-WW-1.16.40.763-user.zip

Asus Zenfone 2 Laser ZE500KL:
http://dlcdnet.asus.com/pub/ASUS/ZenFone/ZE500KL/UL-ASUS_Z00E-WW-12.8.5.222-user.zip
Adb fastboot Tool
USB drivers
Laser ZE550kl

How to Flash Stock Rom on Zenfone 2
In this tutorial I'm taking WW_1.16.40.763 in ZE550KL/ WW_12.8.5.222 in ZE500KL as an example.

Step 1. Once you have downloaded the full stock rom, rename the zip file to WW_1.16.40.763 / WW_12.8.5.222

Step 2. The reason why we did this is, by renaming the zip file the phone gets notified that a new version of software is available to install.

Step 3. Copy the renamed zip file to internal memory of your zenfone 2 laser.

Step 4. Disconnect the phone from pc, you'll get a notification of available update.

Step 5. Install it. Now the phone reboots with the fresh stock rom flashed in it.

If the above method not work try the below one.

How to Flash Stock Rom on Zenfone 2


Turn off your Zenfone 2
Go to recovery mode (Tutorial HERE)
Select apply update from ADB
Connect your Zenfone 2 to your PC with USB cable
Open command prompt in Windows (in the same folder where ADB is located)
Check connection by type adb devices if connection successful will be a list of Zenfone 2 device
Type adb sideload file.zip (file.zip=adjust the filename, asus zenfone firmware that you download earlier)
Wait until the process is complete
Zenfone 2 will flash to clean thoroughly


Hope it helps.