Then Abimelech called Abraham, and said unto him (in the presence of his people), What hast thou done unto us?—identifying himself once more with his people, as he had already done in responding to God (Genesis 20:4)—and what have I offended thee, that thou hast brought on me and on my kingdom a great sin?
The gravamen of Abimelech's accusation was that Abraham had led him and his to offend against God, and so to lay themselves open to the penalties of wrong-doing. Thou hast done deeds unto me that ought not to be done.
Literally, deeds which ought not to be done thou hast done with me. The king's words were unquestionably designed to convey a severe reproach.