To get the Sayonara achievement in State of Decay 2 do the following:
Get one of your settlement characters killed. Don't fight back against enemies and it'll be easy enough.
If you're wanting to do this sooner than later (it may happen whether you like it or not), my suggestion would be to not pack anything in the character's inventory. When they die, you're able to go retrieve what they left behind, but if they die empty-handed and without any items in their inventory, you'll save a step having to go track down your stuff.