פּירוש אויף שמות 9:34

כי חדל המטר, “for the rain had ceased;” seeing that Moses had only promised that the hail and the thunder would cease, Pharaoh said to himself that now he had proof that Moses was only a charlatan. Hence he continued to sin.

ויוסף לחטוא, up until now he had not been a deliberate sinner, not having acknowledged that what he had been doing had been sinful. Now, after having acknowledged this, reneging constituted a deliberately committed sin. This is why the Torah now describes Pharaoh as a deliberate sinner.

וירא פרעה כי חדל המטר, Pharaoh saw that the rain had stopped, etc. The Torah mentions this to show that Pharaoh's repentance already evaporated; he continued to sin almost immediately. When the Torah reports him as ויוסף לחטא, "he continued to sin," this does not refer to his continuing on his previous path; it means that Pharaoh added additional sins to the ones he had committed already. He now made a commitment not to release the Israelites under any terms and conditions.