If you promise or swear to do something evil or sinful, do you have an obligation to fulfill that promise? If you promise or swear to do something evil or sinful, do you have an obligation to fulfill that promise? I know this is probably a silly question, but it's been bothering me. My main biblical source on this that I could find is Leviticus 5:4–5: &quot;Or if a soul swear, pronouncing with his lips to do evil, or to do good, whatsoever it be that a man shall pronounce with an oath, and it be hid from him; when he knoweth of it, then he shall be guilty in one of these. 5 And it shall be, when he shall be guilty in one of these things, that he shall confess that he hath sinned in that thing:&quot;. I think it's implied that you shouldn't fulfill an evil promise either, but I&quot;m not sure. I'd really like some help understanding the verse and with addressing this question.
Scripture teaches God's absolute righteousness and holiness (Is 6, Rev 4:8). Like the priests of old, a Christian's life ought to be &quot;HOLINESS TO THE LORD&quot; (Ex 39:30).

1 Jn 1:3-9 3 That which we have seen and heard declare we unto you,
that ye also may have fellowship with us: and truly our fellowship is
with the Father, and with his Son Jesus Christ.   4 And these things
write we unto you, that your joy may be full.   5 This then is the
message which we have heard of him, and declare unto you, that God is
light, and in him is no darkness at all.   6 If we say that we have
fellowship with him, and walk in darkness, we lie, and do not the
truth:   7 But if we walk in the light, as he is in the light, we have
fellowship one with another, and the blood of Jesus Christ his Son
cleanseth us from all sin.   8 If we say that we have no sin, we
deceive ourselves, and the truth is not in us.   9 If we confess our
sins, he is faithful and just to forgive us our sins, and to cleanse
us from all unrighteousness.

Here we read of the believer's responsibility and privilege to walk in the light - that is, to do what is right. If we sin, we are to confess our sins, but we ought not go back to sin more.
The 10 commandments - as non-optional commands given to Israel - give strict direction, and never take into account if they had promised to sin.
If a believer were to make a promise to do evil, he ought to repent of it and bring it to God for forgiveness by the blood of Christ, as above in 1 Jn 1:9, but he ought not to sin more by fulfilling a promise to do what displeases God.

Jn 8:10-11   10 When Jesus had lifted up himself, and saw none but the
woman, he said unto her, Woman, where are those thine accusers? hath
no man condemned thee?    11 She said, No man, Lord. And Jesus said
unto her, Neither do I condemn thee: go, and sin no more.