I've run into the same situation multiple times and I've found that a bit of roleplay goes a long way. I'm not saying that you don't RP because I know you do (one of the best), but I have been dealing with some REALLY salty off-duty cops lately and they question every single thing - even well known guidebook rules. Sometimes I think they're doing it purposefully to add confusion to the situation in order to get the upper hand. But I digress.
Whenever lethals become necessary I give them an ultimatum and that usually prevents the post-lethal waterworks.
Heck I even did it at the bank once. Granted they didn't get off the roof like I told them to, but nobody messaged me after the fact to complain.