Had to think about this one for quite a while. I have an embarrassingly large video collection, so there are several ticklers that come to mind, but there's kind of a caveat. I don't think I can classify any of them as consistently sadistic ticklers, but... I've seen a video or 2 of these ticklers falling into moments that might count as sadistic. They kind of lose control of themselves a bit and really start laying into the lee, pushing the envelope past what the lee could actually take, much to the lee's dismay (or pleasure).
These aren't rankings, just the order I saw the videos in. I apologize in advance for not remembering the names of some of these videos.
1. Priscilla James - RealTickling/FemFeet/TicklerInBlack
I don't remember the name of the video, but she was tickling a guy. She teased him about being so ticklish and being made to beg by a woman. But there was a moment where she really got him laughing hard and she she got quiet and went in harder. She got so into it she actually, audibly moaned as she dug into his feet.
2. Gia (I think) from Micah's Tickle Torture - Dungeon Maidens
This was probably the only video I'd ever seen of theirs. I was maybe 19 at the time. She laid into this woman the whole video and it was the first time, that I could remember, that it looked like the more the ler tickled, the hornier she got. I think I remember reading the description that she had to be pulled off the lee at one point. Not sure how true it was, but given what I remember from the video, I kind of believe it. She seemed like she was in another world the whole time and all the lee could do was scream and laugh.
3. Tasha or Brooke from Various Videos - Tickle Abuse
You can flip a coin here. I've seen both of them push certain lees too far and yet, I can still tell they are both holding back on camera and if they ever really got a hold of someone, it would be absolutely brutal. Like...if there's no safeword, the lee might actually pass out, type brutal. Lol. I only give a slight edge to Brooke because it seemed like she really did get off on it and it wasn't a performance.
4. Mia - Britney Brooks Gets Effed Up - Tickle Central/Tickled2Death
Title kind of says it all. Lol. I think the description was something like "Mia was in the mood to really tickle the shit out of someone..." Well, she did. Lol. Britney was bound-ish, if I remember. there were two other women in the video, but they literally only helped Mia hold Britney still so she could tickle her half to death. I'm not sure if there was history there or anything like that. I'd seen Mia tickle Britney Brooks before and after, but this one was very different and it felt like one of those moments where if I was there, I probably would have been thinking "should... should I stop her?"
5. Sasha Foxxx - Tickle Central/TickleHotness
There are two instances of her really laying into someone
For Tickle Central I don't remember the name of the video, but she was tickling Lela Beryl who was tied to, what I think was, a massage table. There was a moment where she REALLY got Lela thrashing and didn't let up. I think they had to cut right after that, because Lela had to safeword to get her to stop because Sasha was really working Lela over bad. This was the first time I'd actually seen Sasha. She had developed this character on screen that was really mean. She didn't laugh or smile. she just aimed to tickle the shit out of the lee. she didn't keep up that persona for long, though as she softened her persona up a bit in future videos. She could still tap into it, but she leaned more into the fun side of being a tickler.
The other time was a TickleHotness clip where she played a therapist that was trying to "cure" Kelli Sage's tickle fixation. She was tickling Kelli's foot and, for a moment, there was a point of real panic coming from Kelli, but instead of stopping or slowing down, she held the foot tighter and tickled harder for just a few more seconds before actually stopping. It reminded of that video where Taylor Raz literally couldn't take anymore and started screaming. It was almost another one of those moments, but Sasha stopped before it got that far, almost like she remembered to stay in character and keep the scene going so Kelli could stay in character. It was a fleeting moment, but one that kind of made me think "different time, different place, Kelli would be screaming bloody murder"
6. Scarlett Sinns - Scarlett Sinns
I think someone mentioned her already. I only have a couple of her videos, but I was lucky enough to see the times where she went to town on someone. I don't remember who the lees were. I hadn't bought a video in probably a good 6-7 years, so most of the models were new and I wasn't familiar. I just remember those moments where Scarlett goes quiet. And since it's HD, I can see a lot more, including her eyes. Everyone knows that look a ler gets in their eyes; a look of pure, unadulterated excitement. They have temporarily detached from the outside world and their only purpose in life is to make the lee laugh and thrash. she gets that look. It's almost like, for a solid minute, she forgot she was on camera and is just letting her body react. Love that look.
A few honorable mentions:
Kasie from Tickling Paradise (and Scary Movie, fun fact). She aimed to tickle poor Taylor to death. Lol.
Veronique from FrenchTickling. There's a video that I lost of her tickling Helene. The male tickler usually give Helene breaks because Helen was super ticklish, but Veronique gave none and it didn't appear as if she planned to stop unless directed to. Poor Helene.
