Espionage between civilised countries has fairly civilised almost customary rules since the end of WW2 tbh.

Spies with diplomatic cover simply get expelled/PNG’d, illegals get detained, put on trial and kept for potential horse trading, and local agents (traitors) get dealt with as the country sees fit. Usually by imprisonment, these days.

The status quo of these “rules of the game” being upheld has been the case for ages, it’s in nobodies interest to start breaking those “rules”.