with the ls770, you cant root if your on marshmallow firmware afaik. you have to be on a lollipop firmware.
for the ls770, there is no bootloader unlock. i have a ls770 and have been looking into what the k10 devs did to unlock their bootloader by hex editing and flashing the bootloader partition but just havnt done it yet. if i have any progress ill post it either here or xda. as a result of no unlocked bootloader and lg's tampering with fastboot (dont delete your laf partition, i have done it and it doesnt give you fastboot just bricks download mode if you bork your system partition) there is no twrp for your phone atm.
if you are on a lollipop firmware, then you can root with some of the guides on here that were posted a while back. DO NOT USE KINGROOT ON MARSHMALLOW FIRMWARE!!!!!!!!!!!!!!!!!!!!
kingroot/kingoroot/malware crap might be able to temp root your phone once, but once you reboot it will trigger the secure boot flag as invalid since it will detect a change on the partition structure on your phone and prevent you from booting. im pretty sure you can use one of the tot files provided by asialove and others on xda to reflash, but your mileage may vary. you might have a brick.
thats why i put my ls770 in a drawer and bought an h631, which is the stylo but for tmobile. i got it sim unlocked after 3 months of usage on tmobile, and have had an unlockable bootloader since the day I bought it.
if you want to do root stuff and stick with virgin mobile, see about finding yourself a moto e 2015, otherwise known as a surnia device model. it has an easily unlockable bootloader and plenty of root related stuff available, including a fantastic custom rom selection.
hope this helps!