what does "that’s really the least of it"mean?
“I should have killed you,” Suskind said between his teeth. “I should have killed you after you murdered Lindsay.”
“You’re a fool. And that’s really the least of it.”
He stepped back, just a little, when the first light spilled into the far edge of the area, and shifted his gaze—again just a little—to meet Abra’s eyes.
He’d heard her slip out behind him, out of safety.

Question

what does "that’s really the least of it"mean?
“I should have killed you,” Suskind said between his teeth. “I should have killed you after you murdered Lindsay.” 
“You’re a fool. And that’s really the least of it.” 
He stepped back, just a little, when the first light spilled into the far edge of the area, and shifted his gaze—again just a little—to meet Abra’s eyes. 
He’d heard her slip out behind him, out of safety.

Answer

It means that the other person is a lot worse than just being a fool.  When he called him a fool, he could have said many worse things about him.

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