Here are 5 Bible verses about mercy killing from the Old and New Testaments of the Holy Bible (King James Version), sorted from most to least relevant.
Exodus 20:13 KJV
Thou shalt not kill.
1 Thessalonians 5:14 KJV
Now we exhort you, brethren, warn them that are unruly, comfort the feebleminded, support the weak, be patient toward all men.
Revelation 9:5 KJV
And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man.
Ephesians 5:8-9 KJV
For ye were sometimes darkness, but now are ye light in the Lord: walk as children of light: (For the fruit of the Spirit is in all goodness and righteousness and truth;)
Romans 12:1 KJV
I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.