I wonder if anyone know how to bind messages to certain actions, like killing someone with a crit for instance, if it is possible.
I don't think it's possible to have in game events trigger a message for someone to say. Seems like something that could easily be exploited if it was built into tf2, but you could easily just set keys to specific binds and press them whenever you do the actions you have in mind. The keys I like to use for those kinds of binds are i o p [ ] as well as the numpad (although binding to the numpad is a bit strange. I'd look up the tags for keys in the tf2wiki if you're planning to use numpad to bind).
If the binds for the numpad are the same as they are in CSS, then they're "kp_(function)" (function) being their secondary action, like rightarrow or pgdn.