Why would the police give trouble to, or confiscate the weapon on, a homeowner who shoots and kills a violent home invader, whose weapon is discovered at the scene? Why would they hassle a person who clearly responded with lethal force when confronted by a carjacker?
There was a self-defense shooting here in MA a while ago. A drunk no-good was trying to get into a house. The woman opened the door to see what was up (bad idea). The drunk robber came towards her in a threatening manner. She got her gun, warned him. He laughed at her and kept coming. She shot him once, in the neck. He ran out and collapsed in the street. He survived. He had a very long rap sheet and was well known to local law enforcement. He was arrested at the hospital, charged and convicted.
She was arrested at her home, her License to Carry was suspended, and all guns and ammunition were seized. It took 1 1/2 years before she got her License to Carry, guns, and ammunition returned. She was never charged.
MA may be the "worst case", but the reality is that shootings are seldom black and white. It's hard for the police to figure out exactly what happened, particularly when the perp is lying, the witnesses are contradictory (as they often are, not out of malice, just because people aren't good witnesses), and our hero is so amped on adrenalin that he isn't thinking clearly. So the SOP is to arrest everyone, seize the gun, so that no matter where the investigation takes them, they've got everyone and all the evidence.
Is it possible that you would not be arrested after a defensive shooting? I suppose it is possible, but I suspect highly unlikely.
Plan for the worst case. If it works out better, then great.