asking about 'Purity' achievement and ' Happy Couple' achievement.

[Lucaman]

if i kill Nailsmith and quit the game without saving can i still unlock 'Purity' achievement and doing
' Happy Couple' achievement next?

[Vellutia]

Not sure, as far as I know, Hollow Knight does auto save. I think any events would be saved even tho you didn't rest.

[Omegatherion]

The game does automatically save once you kill the smith. You would have to backup your file, kill him and then replace the file with your backup to get both achievements.

[J-12]

You can backup your save files. Ta-dah.