Character mistake: When Batman looks up at the computer the data of the burglaries, he says they happened "this week", but the dates are June 28, July 3, and July 13. On the screen there's also a typo; "Buaglary." (00:05:00)
Character mistake: When Batman uses his magic computer to analyze Langstrom/Man-Bat, lots of lovely gibberish appears on screen, with some notable typos such as "Bat membaaanes", "Sedum Chloride D", "Mammac higi Index Gat harmal nomeaace at padiotion." Your guess is as good as mine for the last one. (00:21:10)
Answer: Because he stole a chemical from his lab.
But Batman knew that with it, Clayface could permanently retain his shape and Clayface desperately wanted to be Matt Hagen again. In a sense, Batman stopped Clayface from becoming human.