The description of the Overkill achievement says you need to deal at least 8 hearts of damage to get it, but you only get it if you deal at least 9 hearts.

Steps to Reproduce:
1. You have to have the Enchanter achievement, but not Overkill. You can use this command to get Enchanter:

/achievement give achievement.enchantments

2. Give yourself a sword that deals 8 hearts with this command (you have to use a command block, it doesn't fit in the chat):

/give @p minecraft:diamond_sword 1 0 {AttributeModifiers:[{AttributeName:generic.attackDamage,Name:dfjkcn,Amount:15,Operation:0,UUIDMost:564,UUIDLeast:9329}]}

3. Spawn a spider (which has 8 hearts) and hit it. You will kill it, but you won't get the achievement.
4. Use this command to give yourself a sword that deals 9 hearts:

/give @p minecraft:diamond_sword 1 0 {AttributeModifiers:[{AttributeName:generic.attackDamage,Name:dfjkcn,Amount:17,Operation:0,UUIDMost:564,UUIDLeast:9329}]}

5. Spawn another spider and kill it. You will get the achievement this time.