All right I'll go first (clever bastards).
The cruelest thing I ever did to another kid was after a little league baseball game I had played in. Usually, I ended up waiting for my ride home after the weeknight games. I carpooled with another pitcher on my team (I guess we were about 10 or 11 at the time), and our mothers both had other kids to look after / errands to run while we played. The place we played at was remote. There weren't any houses or apartments or stores within eyesight. Basically, six diamonds and the requisite concession stands, dugouts, etc had been carved out of a forest area.
After the games it would get really quiet as people would empty out into their parked cars quickly and take off. Well, not too long after the last cars filed out of the lot, all the lights would get shut off on the little league fields, leaving only the ambient light from the pony league fields to illuminate the place. It was close enough (about 200 yards away maybe and elevated about 30 feet above us) that you could see in the shadows and see well enough to not walk into things, but still pretty dark. If you stood more than say 15 feet away from someone you couldn't see their face clearly.
So this one game, my mom had agreed to bring a third kid along, who lived near the baseball complex. He had just moved into town and I guess our mothers had been introduced prior, and so she was doing this kid's mom a favor as she didn't know the area but wanted her son involved in some activities right away.
Well turns out this kid was one of the shy quiet types who wasn't very athletic. But during the ride over both my friend and I were nice and made small talk. We took him to the place where the teams waiting to play warm up and introduced him to the coach when he got there. The game was played and the kid didn't, basically. And for the most part we all just kind of small talked with him in the dugout, but really didn't pay much attention.
No one that I can recall was overtly mean to the kid; we didn't really have but one asshole on the team and he was a twerp. One of those short kids in school who hangs out with the bully and yaps like a little dog at everyone, because that was the only way he could get away with it and not get pummeled on a weekly basis.
So anyway, the game ends and people file out and the three of us are basically left standing around. We played catch under the lights for a while but I think I threw a wild one and the ball was lost in the weeds just before the lights got shut off. Well our parents always told us to go up to the pony league fields if they weren't there when the lights went out, but of course boys will be boys. We started climbing onto the dugout roofs, breaking into the umpire booth (where all the chalk is, etc.) and just generally being little scoundrels.
Well, at one point (and sorry this is so long) my buddy and I decide we want to see if the concession stand was left open, cuz gosh that would mean free "Big League Chew" (disgustingly sweet bubble gum that came in a pouch like chaw) or maybe some popcorn. So we run off and all of a sudden this kid we're with yells "noo!" We both sort of stopped and turned, wondering what the matter was. He said "I don't want to go over there... you can't see." Well, this was highly unusual to us because obviously we knew the place well and it didn't scare us (kid logic), so why should it scare him?
So we pleaded with him to follow us but he wouldn't. So my buddy says "Common with us or we're going to leave you here in the dark." The kid yelped "Noo!" And my buddy gets that evil kid grin, taps me on the shoulder with his glove and says "common!" Now it comes to it... do I do the right thing and stay with the new kid while my buddy looks for treasure at the concession stand (in the middle of the complex), or do I run with him? Well, this being the Cruelty thread. I ran.
That kid - and I'm not kidding - screamed like he had been kidnapped or something. He was scared shitless, and I just kept running. We got to the booth, it was locked and I immediately felt like a complete fucking demon-boy. I felt horrible because I had been left in the dark when playing outside with my brothers and neighbors when I was younger, and even though I knew it wasn't a rational fear in most places I knew how helpless it felt.
Well, we ran straight back and tried to apologize and get the kid to cheer up. Told him we'd take up to the pony league field and maybe we'd see someone we knew / get a free soda or something ("coke" in southern parlance of course). Well he wouldn't have it. He was just petrified and was now sure that anywhere we tried to lead him, we were up to no good. We even tried to run towards the field at one point, figuring he'd be so scared that he'd run with us (towards the well lit areas), but it was clear after about 10 yards he wasn't moving.
So we went back and tried to comfort him. Told him we ran around the grounds in the dark all the time, that my mom would be by soon, etc. The kid just clammed up and stuck next us like we were his mean old big brothers, and he had no choice but to be near us vs. alone in the dark. Eventually we all got in the car and were all quiet. My mom could sense right away something was up but she didn't know what. When she tried being nice to the new kid and asked him how he liked it, he sobbed a bit when he answered and that was it.
We dropped him off and my mom laid into me pretty good. Said it was my responsibility to make the kid feel welcomed and good about being on the team, and that I had done the exact opposite. Think she grounded me for a week when his mom eventually called, wondering why her kid was so upset. My buddy got the switch I think (whooped, as we called it).
As for the crulest thing ever done to me, there are a variety of bad incidents before and after this one, most involving one of my dipshit neighbors who was older and unpopular at school (but tollerated in the small street we lived on because there weren't enough kids around to exclude him when it came time to play footballor whatever). Bastard even fell on me (ya right) in the snow once and broke my arm. The fat fuck.
But in the end justice was done because he wound up a common criminal with a record straight out of high school, knocking girlfriends up, etc. He did so many bad things to me when I was a kid that when I was older and found out he had been arrested a few times, and all the rest of the sordid details, I actually wanted to go to the county jail where he was being held and say "who's the dumbass now, you fat fuck?" But naturally these things are best left to the imagination and that's where it stayed. Never saw the SOB as an adult....