If you accept that the Torah’s commandments are for our ultimate benefit, a sin is not merely a betrayal toward God, but even a betrayal toward ourselves.  

Our Gemara on Amud Beis quotes a verse in Vayikra (5:21) that discusses the violation of denying under oath monetary claims:

“If a person will sin, make a violation to Hashem, and deny his friend’s claim regarding an object entrusted to him or financial matter or theft…”

There are three different descriptors of transgression in the verse:

1. If a person will sin
2. Make a violation to Hashem 
3. and deny his friend’s claim

Each of these phrases can be understood as referring to a different part of the transgression.  First, there is the theft itself, which is the sin.  Then, there is the denial under oath, which is the violation to Hashem.  And finally, there is the betrayal of the friend’s trust, the brazen denial of the obligation.

Kli Yakkar makes a derash that understands this verse as referring to a different entrusted object - the soul.  God entrusts our souls to us every day.  When we sin, we are betraying His trust in abusing the precious object he has given us, our souls and our lives.

I will add that this betrayal is not only to God but even ourselves.  We are given the opportunity of life, and our bodies, hearts and minds are on loan for a short amount of time.  If we invest wisely, we did ourselves a favor.  The opposite is a dereliction of duty to ourselves.