The Weekly Shtikle - Vayeira
There are many distinct differences between Avraham's experience with Avimelech in Gerar and his experience with Paroah in Mitzrayim in last week's parsha. One can certainly assess the episode in Gerar as having a slightly more pleasant outcome. Rather than being kindly asked to leave as he was in Mitzrayim, Avraham ended up settling in Gerar. Rashi explains simply (12:19) that Paroah was looking out for Avraham's well-being and knew that his people were steeped in immorality. However, it would seem that the distinct actions of Avimelech and Paroah may also be explained by the character of the monarchs themselves. Both Paroah and Avimelech had their entire houses afflicted with a plague. However, when Paroah summons Avraham he exclaims, (12:18) "What is this that you have done to ME?!" Avimelech, on the other hand, approaches Avraham and ask him (20:9) "What have you done to US?!" Paroah was clearly a more selfish individual than Avimelech. Paroah cared only about himself whereas Avimelech showed concern for others.
Furthermore, we find that Avraham presented an alibi to Avimelech and said nothing to defend himself to Paroah. The reason for this seems to be that Paroah did not even give him a chance to answer. When Avimelech asks Avraham why he acted in the way that he did, he clearly wanted an answer and was ready to listen to one. Paroah was not interested in what Avraham might have had to say and did not let him speak. These factors, although not compelling, seem to indicate that Paroah's dismissal of Avraham was not out of Paroah's genuine concern for Avraham's well-being but more likely a sign of his short-temperedness.
Lastly, when Avraham makes a feast to celebrate the weaning of Yitzchak, Rashi writes (21:8) that he invited the "gedolei hador," Sheim, Eiver and Avimelech. Avimelech must have been a respectable individual to be included in the same breath as Sheim and Eiver. Therefore, his good-natured approach to the confrontation with Avraham seems to be a reflection of his character.
Have a good Shabbos.