I have 100 dollars, what's the worst/most cruel birthday present I could buy a friend?

How old are they? As that will completely change what presents fit your criteria.

Glittery slime for their child. It's the cruelest thing you can buy someone and probably costs closer to $10.

Pennies. Individually wrapped.
- Funny story, my uncle did something similar once as a gag gift for me and my siblings. He gave us each a wad of industrial shrink wrap (the kind used to wrap heavy machinery like boats for storage) with like $100 in coins inside. He had actually heat-gunned it to stick it all together. We had to spend the next day pulling it apart to get all the loonies and toonies out

Live insects. A quick amazon search indicates that you could buy 2,000 live crickets or 27,000 live ladybugs.
- I start jumping off walls if I see a single roach. If I got a thousand roaches flying in my face I'd surely burn myself alive.
  
  "This is fine."

An office chair. For that price it will be terrible and too big to do anything with it besides gifting it or throwing it out.

a friend of mine once gifted me some lottery tickets and one fake one. fake one said I had won like 30K but when you read the back to see how to collect there were things like redeemable at yo mamas house etc.
that was probably 20-25 years ago, I still can't fully forgive him.

A glitterbomb and a donation to something they really don’t like. A stripper of the gender they’re not interested in. If they’re italian, throw in a square-shaped pineapple pizza, too.

5x $20 lottery tickets. Most lottery tickets have a 1:3 - 1:5 win rate, so if you're lucky enough he might win literally nothing. Nothing else on this list gives the friend hope. Hope unfulfilled is the definition of cruelty.
- I'd go one further. Scratch off the verification code of each of the lottery tickets scan it. And remove any winning tickets. You don't have to hide this. We're trying to give a crappy gift so if they look and see that you've already scratched them. Your gift then becomes here's $100 of losing lottery tickets. It's beautiful
  
  This is dumb because even if there are winners in the pile, now you can't even cash them in yourself because the codes been scratched. So the joke will be on you when it turns out one was actually worth a million

A goldfish. Just the fish. Ask them to name it on the spot so they have to keep it. Now they need to rearrange their home to make space, and buy a tank and such for it.
Plus, it's cheap, so not much of a refund going on if they decide to just give it back.

If the friend is Chinese or East Asian, buy a black picture frame with a black and white picture of them already mounted. Or buy them a set of knives. Or give them money in a white envelope.
- Knifes are actually kinda good...
  
  Could buy a bunch of cheap gas station knives then put sand in the pivots. Ideal loosing them up first then tighting them after. Maybe wash them to remove oils and let them rust some.
  That might be over board. Especially if the dude is a knife guy. Not sure if messing up a new $100 knife would be worse though.

Assuming you are just a horrible person: Buy them gympie gympie leaves, toss them in a gift bag, and don't tell them what they are but that their present is in the leaves.
The present is incredible pain btw.

There was a company once specilized in delivering boxes full of the shit (meaning feces) of your choice to your chosen special person, it was called something like "shitexpress"