Why thief???
You give MVitaliyB money, and hi not return their... BAD PERSON - not thief!!!
Malfunctioning ATM takes guy's debit card. Guy at ATM parks and goes into the office to talk to tellers. You pull up and see his account still up - and there's the "Quick Withdraw - $300" button on the screen. Is it not theft if he "gives" it to you by accident when you click the button to take what you know is his?
Not theft exactly, more like fraud committed against the bank. The money in the ATM belongs to the bank, not the account holder. You're tricking the bank (in the form of the ATM, though it would be the same with a human teller) into handling you $300 by claiming to be someone you aren't. Even though the machine dispenses the money, it doesn't become yours, because you have no contract with the bank. Even in cases where there would be an implied contract, any contract based on fraud is void. Since the money still belongs to the bank, if you walk off with it then you have, at that point, committed theft (against the bank, not the person whose account was on the screen).
Whether the account holder is liable for the bank's loss of $300 is a matter between that person and the bank. Leaving their card at the ATM could be a violation of the contract governing the account, and the contract may stipulate that they will reimburse the bank for losses owing to insecure handling of their card or other authentication credentials.