I tried to carefully explain why Chaim ben Pesach's type of reprimand is different than the one I am opposing here. I do not know how to explain myself any more clearly, so all I can suggest at this point is to re-read what I have already said.
As far as whether or not keeping Sabbath is one of the worst things to violate, the answer like the answer to just about any such question is, Yes and No.
Keeping the Sabbath is considered one of the signature practices of the Torah observant Jew. In fact, it is one of the three ways that Jews in the modern world determine if their fellow Jew can accurately be called an Observant, Torah Jew. (the other two ways are eating kosher, and following the laws of family purity). A Jew is not even permitted to eat at the home of somebody who may keep kosher perfectly, yet does not keep the laws of the Sabbath.
On the other hand, I have to return what I said earlier, and that is that how we treat one another is far more important than any ritual laws we might follow. A Jew who drives on the Sabbath, or goes online on the Sabbath, but is a kind, sensitive human being, is a Jew far more worthy of our admiration than is a Sabbath observant Jew who does not even behave with minimal moral decency.
Also, there is an expression that says that our spirituality is not where we are on the spiritual ladder, but whether we are moving up or down on that ladder. A Jew with no Jewish background whatsoever, who decides to drive a car half way, and walk the rest of the way, to the synagogue on the Sabbath, may be committing a much loftier act than his fellow Jew who walks the entire way to the synagogue. It all depends on the spiritual effort one makes. As it says in the Chapters (Ethics) of the Fathers, "According to the spiritual effort, is the heavenly reward."