I solo slooped over to an active crows nest fortress only to find a brig already there. Rocking the offer alliance flag, I slowly sailed around the island, making sure to sail around slowly, unthreatening, and throwing up "LET'S FORM AN ALLIANCE" multiple times. Initially the brig seemed friendly. I didn't even want to stay at the fort, I was just looking for an alliance.
After a minute of repairing the holes the towers made, I start to pull my sloop up next to their brig in a spot out of the range of the towers. That's when I heard a prepubescent kid say "just shoot him" and I was immediately hit in the face by a cannonball and killed.
I respawn with a new ship and sail back. This time acting friendly, but not really. I park my sloop on the opposite side of the fort and throw up a couple more "LET'S FORM AN ALLIANCE" but instead of running down and repairing like they probably thought I would. I jumped onto the fort, snuck around looking for the closest keg, and used it to sink their empty ship. I don't even know if they realized it sank as they were picked off by skeletons one by one.
Eventually the brig came back but with only one crew member. As he hit his anchor, he also hit a keg I had floating in the water. Instead of repairing, he abandoned ship and started to swim to mine until I sniped him in the back.
Kindness can and will be rewarded, but unnecessary aggression is punishable and won't be taken lightly. If they had just allied with me, they would have completed the fort and probably made a bunch of money off my chickens. Instead they weren't nice and they lost everything.