The Colts left in the middle of the night because the State of Maryland threatened to seize the team using eminent domain. The legislature passed and the governor signed a bill to take the team, but the Colts had left town a few days before.
You're right about how it was disgusting and underhanded, but it wasn't the Irsays.
The Irsays had plenty of time to do whatever was necessary when the team was in Baltimore. The Irsays were underhanded owners of the team from day one. Do you think that there was a bit of a dust-up in the first place causing all the problems that led to the state’s threat? Yes, there was.
That they took artifacts that belonged to the BALTIMORE Colts and the people of Baltimore.