Mat 9:4 Why do you think evil in your hearts?

The term translated as "think" is enthumeomai, which means "to ponder," "to notice," or "to infer." Here, the sense it that the scribes who think that Christ is blaspheming are quick to infer things about others.

What do they infer? The term translated as "evil" is poneros, the adhective most commonly translated as the noun "evil" in Matthew. It means "oppressed by toils," "worthless," "base," or "cowardly." It doesn't mean evil in a malicious sense (except as it has been translated in Matthew). Even in Matthew uses another term for that type of evil, kakia.

My sense is that Christ is saying that the scribes are quick to see base motives on the part of others, as well being burdened in their hearts.

Why does the scribes accuse Christ of blaspheming for saying that the invalid's failures have left him? Because only God can reward or punish or know the future. At the time, people thought that afflictions were punishments from God. Those who spoke against those afflictions were speaking against the will of God.