Can someone act immorally if they both understand what "moral" means and (robustly) believe there is good reason to be moral?
Certainly. Simply knowing the definition of the word, or even knowing the "right" thing to do in every circumstance and also knowing there is a compelling reason to do the right thing in every circumstance does not necessarily force a person into a life of moral actions without adequate moral reasoning.
I'm asking because I'm (idly) wondering whether the agents of immoral actions always lack moral reasoning: such that the existence of moral theory is entirely irrelevant to their choices (whether or not that is the state decides to put up with it, or punishes / educates them etc.).
A good (yet silly) example of this comes from TV sitcoms - it's common for the lovable, well-meaning characters to know what is the right thing to do and to really want to do the right thing but fail hilariously because of their own irrationality in sizing up situations.
I'd argue that this is the most sensible theory of how immorality usually works in the wild (i.e. no maniacal dictators, no sociopaths, etc.): people know what the right thing to do is and know that being moral is better than being immoral, but can't put two and two together to realize that they are in a situation where their moral reasoning is important. Instead, people act off of impulse, necessity, or short-sightedness, short-circuiting they reasoning process with heuristics that provide "easy answers" to the moral quandaries life demands them to solve (often on short time frames).