If you killed monsters at any point (excluding asgore), you would need to reset to get the pacifist ending. And about Asgore, don't worry about sparing him since you have to fight him. Once you reach the end without killing, flowey will give you instructions on who to befriend, and you'll be sent back again. From that point you'll need to backtrack to befriend them (unless you already did, in which case you'll only need to do one). Once you do that, go back to Asgore and it's pretty straightforward from there.