My M1.9 site is still live and I am building a new M2.x site so I need to migrate my data to the new site. I wonder whether my M1.9 database will be gone after migration, do I need to back up it using this tool?
Magento 1 database will remain same as before, data migration tool only migrate data nothing else. And If you don't have data backup before any operation then It is going to like this
Also, please try first in your staging environment Instead of live environment. Further more you can read more about this from this link