You were PKed for 2 reasons, both of which occurred in a short period of time.
A long term made man vouching that you lockpicked inside of our own hotel
Carrying and waving a firearm, with a scratched serial number and while not holding a license, around our most active property
First is obviously an issue and the second is blatantly against family rules, as well as just not being how we want/expect anyone in the faction to act. Especially at a property that is at times heavily observed by police, although I think that should go without saying considering you seem to understand the size of the faction. A large part of this life is keeping anything illegal that you do lowkey, out of the view of outsiders in order to prevent attention/heat being brought to the family.
These 2 things in such a short period of time led to one of the Don's trusted making the decision to whack you out of the faction.
Here is the agreement you accept whenever joining a faction, warning of what may come.
Ultimately it just didn't work out and I wish you luck in whichever faction you end up staying in.