Isn't it obvious? Do you not know what a sadist is?
Sadistic people hurt others for their own enjoyment. A "tickler sadist" would therefore be someone who used tickling as their method of inflicting hurt.
And before the trolls come in and say that tickling is harmless and it's not possible to hurt someone with it, while it's true that no lasting, actual physical damage is done, it definitely can inflict emotional distress and suffering... and to some sadists that's just as good, if not better than regular pain.
At least two of my girls were ticklish enough that, if I were a sadist, I could have absolutely wrecked them. Some doofus who pokes you in the side to get a "tee hee" isn't even in the same league.
What you're probably expecting is for someone to say that you can't be sadistic unless you tie them down and torture them for hors withot a safeword. That's not true. One of my exes had an abusive ex of her own, and he would hold her down and tickle her whenever she didn't do as he said. He knew she hated it and he used it as a weapon against her. That's a sadist.
And personally, I don't think consent has anything to do with it. If the victim wants you to hurt them, that doesn't make a sadist any less sadistic, just that it's not abuse. Unless the sadist only gets off on the involuntary aspect of it, that is. I guess a sadist and a masochist would be a dream couple in most cases.