That was a sh*tty ending
Why did Stan just let them go? That makes no sense at all. Even if he takes all the half-truths and lies by omission that they tell him in that last meeting at face value -- which he has little reason to -- just arrest them and let the prosecutor sort it out. Stan basically knows that they killed all those people in Chicago, he's highly suspicious that they committed other murders, but instead of arresting them, he essentially commits high treason by letting them go. And it doesn't even make sense in the grand scheme of things -- even IF Stan wanted to support Oleg/Philip in their efforts to keep Gorbachev in power (and he doesn't, he said so himself), Philip's/Elizabeth's arrest wouldn't make much of a difference there. Just let the feds sort it out. But don't let two suspected mass murderers just walk.
This show has always been prone to underwhelming season endings (the previous season being a prime example), but this sort of takes the cake. :\