The company I used to work for was fairly small (5 core staff including the owner, 2 part time). For the longest time, I had been looking for work but had been very hesitant in leaving I had a senior role within the company and felt scared my leaving would have a big impact on the company which I cared a lot about (my boss and I had and continue to have a fantastic relationship). However, I had passions for other things in the company (which the whole team knew about, including my boss) which I was never given a full chance to move into, despite being very vocal about it. About 3 months ago, we had hired a new core staff person, and he got given the title (and responsibilities) of the type of position I had been interested in (a position was explicitly created for that person in that role and title). I actually like the new guy and am happy he got what he wanted, but was very angry at the owner for not letting me move into that position. I got very pissed and started sending out resumes and really didn't care what would happen to the company. And I didn't feel it was enough.
I slowly stopped work on client projects and tickets. 2 other senior staff didn't know what was happening (they didn't realize it was me) and I started planting the seed in them that they should move on. And one by one, I left for a new job, the other senior staff quit (despite having no other job lined up) after a few weeks, and just recently, the final one also announced quitting. And given there are no more senior staff at the company, the owner is deciding to close shop. No one was wholly satisfied there, but I know my actions completely led the charge in making the others quit. I didn't expect the owner to close shop but he did and I completely regret that.
The owner doesn't know I caused all of this and we still have a good relationship. I know he'll start something new and recover. But the new hire got royally screwed alongside the 2 part timers, and I am an asshole for doing so.