Self-defense and defense on the behalf of others.
It's curious these questions keep coming up since it's been well documented that JN was making threats of violence, saying he'd take a bullet, didn't care if he got a life sentence, would kill all the MF'ers, etc. Then ripped off his jacket and threw it to the floor.
Not a single person on that train car knew if he'd actually act upon what he was saying, and DP and the 2 others that attempted to restrain him, didn't wait to find out. It ended tragically with the loss of JN's life, but the threats of violence are fully documented.
Interestingly, hindsight being what it is, we the public now do in fact know that JN did have a history of acting out his violence tendencies on others. Realistically, we have no idea what JN might have done if DP hadn't stopped him. It's possible he would have just sat down and did nothing. It's also possible he could have punched another elderly man or woman in the face like he'd done before. JN was a violent person, and that is also very well documented.
jmo