From a software perspective adding a kill switch is needlessly adding a potential vulnerability.

Of course they would test against all imaginable vulnerabilities. It is a weapon after all.

And of course they would make all the needed software tests in a safe environment where the switch doesn’t really destroy things.

need spare parts and software updates

As I said, no direct profit from the kill switch. But the possibility of indirect profit: for every destroyed weapon, there is the potential to sell a new one later.