Yet another case of suspected chickens killing each other being proved false. In reality, it almost never happens that chickens kill each other. If, in the rare instance they do, it's almost always accidental or as a result of an infection following injury after a pecking order altercation or...
Well, goshawk then.
You said there was standard size poultry mesh. Was that on a wall exposed to outdoors where a snake could crawl through? If so, a small mink or other weasel could also squeeze through.
Game cam!
You really do have a "who-dunnit". The best thing would be to set up a game cam. I have never done it, but I hear they are very inexpensive and easy to use and download to your computer.
If you just have standard poultry mesh between a predator and your chickens, that isn't small enough mesh...