Frank Castle, aka the Punisher, is a mass-murdering antihero who kills criminals by the dozens, but he secretly harbors a die-hard admiration for another superhero: Captain America. Though the two Marvel characters couldn't be more different, Castle's adoration of and respect for Captain America runs so deep that he will willingly go along with Steve Rogers regardless, of his own thoughts or feelings. This affinity is so strong that Frank Castle is even willing to take a savage beating from Captain America... without even fighting back.
The Punisher is infamous among Marvel characters for a single reason: he does not ascribe to the "no-kill" policy of many heroes and will massacre criminals without a second thought. He does not believe in rehabilitation and has killed everyone from murderers and mob bosses to - in extreme cases - common thieves. This violence stems from his tragic past; ever since his wife and children were murdered in cold blood, he's waged a one-man war on crime and mob activity. But as dark and violent as Castle may be, he still admires Captain America - one of the most morally pure heroes in the Marvel Universe.
In 2006's Civil War crossover event, during the debate concerning a Superhuman Registration Act, Frank Castle is firmly on the side of Captain America's anti-registration guerrilla fighters. Two C-list villains, Plunderer and Goldbug, want to join up too - they don't like Iron Man's burgeoning police state-tactics and wish to form an alliance with Captain America. The Punisher doesn't hesitate: per his policy, he immediately guns them both down, murdering the villains in cold blood. Captain America is appropriately furious.
Even at the receiving end of a savage attack from Captain America, Frank Castle still refuses to fight back. Castle likely knew what Captain America would think about gunning down helpless people - even if they were villains - and he did it anyway. The Punisher is an uncompromising individual in many ways - but he still doesn't see himself as a hero, and places Captain America in a category far above his own station.
In Punisher #13 by Matthew Rosenberg and Szymon Kudranski, the Punisher is met by two corrupt police officers who proudly display his logo on their squad car. To the surprise of the officers, he tears it up. "We're not the same. You took an oath to uphold the law. You help people. I gave that up a long time ago...You boys need a role model? His name is Captain America, and he'd be happy to have you."
Frank Castle looks up to Captain America because he is everything the Punisher is not: forgiving, willing to compromise, and someone who sees the good in all people. The Punisher does not. Forgiveness is simply not in his nature and yet he continues to honor Captain America. In this respect, the Punisher is a tragic figure; a man who falls short of greatness due to his own demons and weaknesses, and knows he will never become the one he idolizes most: Captain America.