It would likely depend on the specific individual, but generally a low functional IQ does not render someone a halachic shoteh ("insane" or "irrational.") I assume that was your question about Down Syndrome was effectively asking about a low IQ.
We have the concept of oness Rachmana patrei, G-d does not hold us accountable for what's truly beyond our control. Hence a person is exempted from mitzva obligations if they lack the cognitive ability needed for their most basic performance. I very much doubt that was the case in the incident you described; and I hope that individual is getting the help he needs (it seems he's crying out for it).
There is the famous, famous responsum of the Chasam Sofer (or was it one of his descendants?) about institutionalizing an individual such that only non-kosher food will be available, with the goal of increasing their ability to function. Rabbi JD Bleich proves conclusively (yutorah mp3 available) that the case concerned a developmentally challenged individual (who is obligated in most mitzvas), not a severely insane one (who is not).
After using the term shoteh (exempt from mitzva obligation) 50-something times in his code, Rambam then writes:
ט,ט [י] הפתיים ביותר שאינן מכירין דברים הסותרין זה את זה, ולא יבינו ענייני הדבר כדרך שמבינין שאר עם הארץ, וכן המבוהלין והנחפזין בדעתם, והמשתגעים בדעתן ביותר--הרי כל אלו בכלל השוטים. ודבר זה, לפי מה שיראה הדיין, שאי אפשר לכוון הדעות בכתב.
The exceedingly simple, who cannot understand that both A and B cannot be true as they conflict with one another, are considered shoteh and therefore disqualified from serving as witnesses ... and such matters must be judged individually by the judge and can't fit an easy written criterion.
Rabbi Moshe Feinstein says that for the first 50-something uses of shoteh (which are all about his mitzva obligations in general), Rambam didn't have to define it -- it meant someone who you know it when you see it, someone completely crazy and disconnected from reality. It was only with regards to serving as a witness that the severely developmentally challenged is excluded.