Avraham is depicted as a man of immense kindness. He risked his life to save Lot, he hosted unknown guests and even tried to save Sedom from destruction. His actions starkly contrast with those of Noach, who upon being told by G-d that He was about to destroy the whole world, did not utter a single word of protest (Zohar, part 1, p. 67b). Even the episode of Sarah and Avimelech can be construed as an act of kindness on Avraham's part. When Sarah was abducted by Pharaoh, Avraham asked her to pose as his sister so that they would treat him well, and through her efforts his life would be spared (Bereshit 12:13). In the case of Avimelech, however, no justification for this deception is given: Avraham simply announced that Sarah was his sister. This change is obviously done to teach us something. Avraham's love for Sarah was in a sense fraternal, a pure and idealistic love that involved no feelings of jealousy or possessiveness that are so often present in a husband-wife relationship. Avraham Avinu had no selfish and petty reasons for keeping her for himself. She was thus theoretically free to go and become the wife of Avimelech if she chose to do so. Such a choice was obviously a legal and moral impossibility and Sarah did not entertain it at all. From Avraham's point of view, it was possible (See Talks by Ha-Rav Tzvi Yehudah on Parashat Vayera summarized by Rav Ilan Tor). Kindness stems from the ability to see the Divine light and content that exists in every creation. And thus Avraham, who saw the G-dly illumination in all creatures, loved them all.