I'm going the route of the Ultimates line for Marvel. In the Ultimate Avengers series Nick Fury recruits the Punisher as an Avenger...seriously it happened. In the story arc "Crime and Punishment" Punisher shows up, then kills 35 people in the first 8 pages. Then Captain America shows up in disguise as a Punisher worthy target and takes Frank Castle down. You might think how could a Marvel Hero go around killing so many guys and still get to be an Avenger...well in the words of Arnold Schwarzeneggar..."Dey were all Bahd". Basically Ultimate version Nick Fury gives Punisher a get out of Jail free card if he joins up with a black ops version of the Avengers. Someone they can use that will do the dirty work that the regular team can't get away with publicly.
So there you have it, the Punisher was an Avenger.
Avengers April - the Punisher - 286
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