How can we thank God?

David, who said to Abiathar “I am responsible for the death of your father’s whole family”

(1 Sam. 22:22), gave thanks to God (Ps. 52:9) even though he had brought about

the death of Abiathar’s father Ahimelech the priest and eighty-five men who were the linen ephod (v. 18).

I don’t understand this.  I don’t understand how David was able to give thanks to God?

If I was David, I think I would feel very guilty. 

So I am wondering: ‘How could David give thanks to God in that kind of circumstance?’

Maybe the reason is because David was relying on God’s lovingkindness (Ps. 52:8)

And maybe the reason is because he trusted in the lovingkindness of God (v. 8).