To the extent he is remembered today, it is as a villain for his role in suppressing the Spartacus revolt. Inheriting a craven army that had run from Spartacus' slave army, he decimated them, i.e., chose one out of ten to be killed and forced his fellow soldiers to do the killing, a thing that had not been done in many years. McCullough, it should be noted, makes this even worse than it was. Where Plutarch says that Crassus picked the 500 worst offenders and executed one out of ten (total: 50), McCullough has him decimate the whole legion of 7500 (total: 750). Crassus makes clear that his war against Spartacus and his followers is one of extermination. When the starving slaves ask his terms of surrender, he says he has none; his goal is to destroy them. And when some 6,600 are taken alive, he has them crucified along the roadside and does not break their legs. While Pompey, equally rich but more respectably so, gets his faction in the Senate by bribing them, Crassus lends Senators money without interest and then calls in favors by threatening to call in the loan.
Crassus was described has having hay on his horns. Though horns are often associated with a cuckold, the meaning in this case is not sexual. When an ox was prone to gore, Romans would tie hay on his horns to show that this beast, however, docile and placid he might seem, was dangerous. McCullough takes this to mean that Crassus seemed outwardly placid and bovine, but was actually no one to be messed with, as many a slave or soldier learned the hard way.
Oh, yes, and he is a close friend and ally of Caesar. That is historically accurate. Caesar was in constant debt because of expensive personal and political habits, and Crassus regularly bailed him out. Their alliance does not say much in favor of Caesar! Which leads to my final character study: the hero of the series.
*There are probably not many ways a Roman could get rich that modern sensibilities would approve of. Large land holdings, the most approved form of wealth, meant squeezing out small holders and increasingly meant the use of slave gangs chained together and locked in barracks for the night. Mining used an even more brutal form of slavery. The point is not so much that these forms of wealth used slaves, which was an unavoidable practice at the time, but that they were especially badly treated. If anyone in the novels so far made what we would consider an honorable fortune, it would actually be Quintus Servilius Caepio, Jr., who invested his vast fortune in gold in shops and smithies to equip Rome's armies, spreading great wealth wherever he went. And where did he get all that gold? Honorably enough, he inherited it from his father, QSC, Sr. And where did QSC, Sr. get the gold? He stole it on its way to the Roman treasury, killing hundreds of Roman soldiers guarding it. Junior had no role in his father's crime, but his wealth was nonetheless generally seen as tainted.
**Some libertarians would presumably defend his actions, saying that people are better off selling their houses at an inadequate price, saving their movables, and being able to leave in an orderly fashion than having their house burn down and losing everything.
***Of course, no libertarian would defend that.