Asus Zenfone 2 users can now unlock bootloader on their smartphone. Unlocking the smartphone will allow you to root the smartphone and install custom ROM.

Step-5: Now reboot the bootloader using "adb reboot-bootloader" command. Step-6: Use fastboot to unlock the bootloader by executing "fastboot devices" command.

But I went another way, using Chainfire’s CF-Auto-Root for the Nexus 5. This is another utility that will unlock your bootloader and root your device in one fell swoop.