I don't find the message "creepy" at all.
It says BYE BYE and has 2 smilies drawn in the dirt on the tailcone.
There is no threat, no warning.
The flight attendant scare looks like some kind of "group dynamics" to me, with emotions boiling over.
If the pilots deemed it safe to fly in consultation with the company and had the area reinspected with no findings, I find the insubordination out of place.
“Our flight operations, safety and maintenance teams appropriately investigated and determined there was no credible security threat,” United spokeswoman Christen David said in a statement. “All of FAA’s and United’s own safety procedures were followed, including a comprehensive safety sweep prior to boarding, and the pilots, mechanics and safety leaders deemed the aircraft entirely safe to fly.”
But given, that they might really have been afraid for their lives, even for reasons out of being overemotional, firing them all imediatly is way to harsh. Kinda shows a little about the company culture on dealing with stress incidents.
Whoever did this should better have written "Wash me!"...