I don't buy the stuff about it being a bit of fun, or a misunderstanding.
People who really tickle, until the tickled party is gasping and howling and has lost control altogether ARE getting off on the pain of it. Of course they are. They're inflicting extreme discomfort and desperation for the purposes of their own amusement - I'd have thought it was obvious!
Someone did it to me once to torture me when I was about 22 - friend of mine I'd just made up with after a very very messy feud, and whose feelings for me were unreciprocated. He was a twisted little fucker and tickled me knowing full well I hated it for about three, three and a half minutes. I have been raped, sustained some incredibly nasty injuries, had a termination go badly wrong, been called a worthless cunt and walloped silly by my father... plenty of pretty shite things and I can honestly say that that tickling ambush ranks up there as one of the worst experiences I've ever had. I was begging and hollering and shaking and hyperventilating and he just carried on and on. He only stopped when I called out for his rather imperious flatmate in desperation.
Believe me I'm a hard-faced cookie but it was fucking horrendous. Painful and humiliating. He admitted to me afterwards he was deliberately torturing me because he was "feeling resentful and angry towards me" and confirmed that people who do tickle people until they are writhing in agony are absolutely getting a sadistic kick out of it, which is obvious anyway.
My current DP tried it once, two or three years after the above event when we were newly together. He only kept at it for 10 seconds or so but I wasn't laughing, I pleaded with him to stop, and he didn't. The fear took over and the button was pressed. He ended being elbowed in the sternum, bitten, threatened with far worse and told that if he ever did it again, he could jump on his bike and fuck off for good.
He never did it again.
I implore people not to do it unless their victim has expressly told them that they want it.