And even keeping track of "Pain Killer usage" is only indirect (via keeping track of "Morale restored so far").
If that means that _moraleRestored variable gets altered even when morale goes up by shooting aliens, then I admit that my idea is screwed. But if not, then
Or in other words: "the game does not keep track of pain"
In the end it all boils down to definition of a word "pain".
Your definition is "morale, that is substracted by damage".
My definition is "morale, that is added by Pain Killer usage".
Your definition is correct.
My definition is how "pain" is actually tracked by the game.
There is less "pain" in my definition. But there is enough "pain" to make Pain Killer useful if implemented as morale cap. At least for a proof of concept reasons.