He's an Edgelord Ex-Soldier with lots of guns who kills anyone who commits any crime with extreme prejudice, you now know literally everything there is to know about the Punisher.
As for his Plot Armour? "Punisher Kills the Marvel Universe" is a thing. That's right, Angry man with Guns™ can kill Omnipotent Aliens, Inter-dimensional Tyrants, and Literal Gods.