Someone hasn't done their research

Rip-Off and Parody are two different things.

Parody implies being mimicked for comedic effect. Which is what Deadpool's creation was. Deathstroke and all straight-laced bad@ss characters, were popular at the time. So someone thought It'd be funny to make a character who was a bad@ss mercenary who was funny instead of dark and gritty and serious.

Except unlike Deathstroke who has basically been the same generic bad@$$ character and have never really evolved beyond that. Deadpool actually became his own character, with personality and flaws and motivations. He evolved into a tragic sad clown nominal-hero who wants to do good and be a decent human being but is constantly pushed and pressured into being destiny's garbage man and uses humor as a coping mechanism to hide his own insecurities.