[Nexus 6P Q&A, Help & Troubleshooting] TNS201:
http://xdaforums.com/nexus-6p/general/guides-how-to-guides-beginners-t3206928

This will fix your problem. See link above ^^ Your bootloop sounds like you didn't do step 5 and get the modified boot.bin for root. http://xdaforums.com/apps/supersu/wip-android-6-0-marshmellow-t3219344. Make sure you follow step 5 in the guide in the link below. Also another thing that can cause bootloops is you let TWRP install SU/Root, if it asks upon exit, make sure you decline. I was messing with my phone for hours friday night because of bootloops but you just need to folow the link below, start at step 8, don't relock your device, then test to see if you can boot into Android again. Upon confirming you can boot android again, start the guide at step 2 once you get things working again. Install TWRP, then step 3, then step 5 you want to make sure you follow 100% so you don't get bootloops. Make sure you flash Chainfires boot.bin as linked, then make sure you have put the SUPerSU zip on your internal storage and you flash that in TWRP install. Then you should be golden. Make a nandroid backup of your system once you get root installed properly.

Go to step 8. Get the factory image https://developers.google.com/android/nexus/images?hl=en Don't relock your device though like it says to do at the end of step 8. Also you you might have to change some commands around in step 8 if you are using a different build when sending commands in the bootloader. bootloader-angler-angler-01.31.img radio-angler-angler-01.29.img are the two files that will differ from build to build. Just change the command to use the proper filename.

Download the appropriate factory images for your model here.

Unzip the angler-xxxxxx-factory-xxxxxxxx (x's replace variables) into a directory on your PC (for this guide we'll use C:\angler\).

Once unzipped you should have a folder named angler-xxxxxx (x's replace variables). Open it and move the following files to a new directory named C:\angler\images\:

bootloader-angler-angler-01.31.img

radio-angler-angler-01.29.img

(Please note that radio and bootloader naming changes from build to build so the filenames and below commands will change with them)

Within C:\angler\angler-xxxxxx\ you'll see another zip archive named image-angler-xxxxxx, unzip the contents of that zip into the C:\angler\images\ folder we created before. Now within C:\angler\images\ you should have the following files:

android-info.txt

boot.img

bootloader-angler-angler-01.31.img

cache.img

radio-angler-angler-01.29.img

recovery.img

system.img

userdata.img

vendor.img

Boot into the bootloader and connect your phone to your PC via usb cable.

Open a command prompt from within your fastboot folder (navigate to where you have fastboot.exe located on your PC, shift + right click anywhere within that folder, select open command prompt here), enter these commands to flash:

Code:

fastboot flash bootloader C:\angler\images\bootloader-angler-angler-01.31.img

fastboot reboot-bootloader

fastboot flash radio C:\angler\images\radio-angler-m8994f-2.6.28.0.65.img

fastboot reboot-bootloader

fastboot flash boot C:\angler\images\boot.img

fastboot erase cache

fastboot flash cache C:\angler\images\cache.img

fastboot flash recovery C:\angler\images\recovery.img

fastboot flash system C:\angler\images\system.img

fastboot flash vendor C:\angler\images\vendor.img

The following command is optional as it isn't usually necessary:

Code:

fastboot format userdata

Please note that formatting userdata will (obviously) wipe all user data from the device, this includes everything on your internal storage (images, music, files, etc), apps, and system settings.