Person A is bein chased by person B who is carrying a knife. Person A has good reason to believe that if person B catches him, his life will be in danger. As person A runs away, he sees a convertible car with the keys in it. The top is up. He does not know who owns the car. Can he get into the car, start the engine and legally drive away to safety?
I am in the United States.
Generally, the defendant must affirmatively show (i.e., introduce some evidence) that (a) the harm they sought to avoid outweighs the danger of the prohibited conduct they are charged with; (b) they had no reasonable alternative; (c) they ceased to engage in the prohibited conduct as soon as the danger passed; and (d) they themselves did not create the danger they sought to avoid.
The necessity defence is recognised in most common law and civil law jurisdictions and in international law. In common law jurisdictions, the “harm” must be some sort of clear and imminent peril to life and safety, such as the situation you describe (although B needs to prove all the elements). In civil law jurisdictions, the peril can be more distant such as the successful defence in 2020 in Switzerland of protesters using it where the threat was climate change.
Short answer? No.
Different States have different statutes on taking and/or driving a vehicle without the owner's consent (for example 14-102 in Maryland and 10851 a VC in California [commercial rather than government sites linked]), but law against taking without consent seems present across the US. It's worth making the distinction between theft (intent to permanently deprive) and taking without consent (intent to temporarily deprive).
In this case, there could be grounds to argue intent - that depriving the owner of use of the vehicle was incidental to the intent to escape.
In similar cases, a prosecutor may decide that intent to deprive was not established, or that prosecution is not in the public interest. A court may find Person A not guilty, or that there was significant mitigation of the offense - the website linked for California specifically mentions duress as a defense. But none of those are the same as the action being "legal".
The scenario is at best only telling half a story. The prosecutor and court are likely to be far more interested in how long Person A drove the car, and what attempts they made to return it to its legitimate owner.
