The best way to de-escalate is to leave. The best way to show an attempt at de-escalation is an attempt to leave.
That being said, every situation is unique, as is every location. Be aware that if you make it known you have a gun, the BG may very well pull his gun out first, shoot you and take your gun to use in his next crime. Letting it be known that you might be armed is a gamble you may or may not want to take.
Where I live, it could be called brandishing.