There's always this. "Is it okay in this highly unusual circumstance where I set it up so that obviously the right answer is to behave terribly?". The world is replete with these questions.
The problem is that these quandaries are then used to justify shitty behavior in other circumstances. What percentage of cop beatings save lives versus total cop beatings? 0.2%? 0.02%? What's the point of your question? What does the knowledge that you will be beaten if a cop suspects you of a terrible crime do to your state of mind? To the safety of police officers? To society?
I'm concocting a situation that I would like you to consider. Answer, please, with just as much gravitas. Let us imagine a society where beating or harassing anyone is absolutely taboo. In this society, no one need ever worry about physical violence. A woman could walk naked from Chicago to California never once being harassed. An elderly man could walk anywhere carrying a fat wallet of cash without fear of physical violence.
There is the rare and occasional thief, however. Now, one of these thieves steals a car and leaves a child baking in the sun in much the circumstances you describe. Beating the thief is the only possible way to save the child from a grizzly, horrendous, agonizing death, screaming alone for her mother.
But, once done, the beating breaks that taboo. Those police officers are a bit more likely to use violence to get what they want. It's a bit contagious, too. Other people begin to use violence. Bear with me, here. Let's say, because that taboo is broken, 100 more people are beaten over the next decade, and 1 or 2 of them die from their injuries. None of them "deserve" it.
Would you say it was worth it, to beat the man to save the life of that child, knowing that 1 or 2 people die and 100 are beaten, directly because of that?