When we say that Booth and Lincoln in Topdog/Underdog are victims of circumstance, we don't mean that they lack agency or that they cannot be held accountable for their actions. It simply means that the choices that they make take place against a social environment created by other people. Being victims of circumstance doesn't mean that Booth and Lincoln should be excused for what they've done, but it does mean that their behavior, bad as it is, can at least be understood.
The biggest single social factor determining the brothers' behavior is their abandonment by their parents. Their mother and father each took off to be with their lovers, leaving behind an “inheritance” of five hundred dollars. Given what happened, it's not exactly surprising that Lincoln and Booth have turned out the way they have. Deprived of parental guidance, they have never fully matured as men, with all the responsibilities that being a grown man entails.
To a large extent, then, Booth and Lincoln are products of a troubled upbringing. Whether this elicits sympathy is largely a matter for the individual. But those who do sympathize will see in all the theft and the card games and the shirking of responsibilities—and, finally, the murder of Grace—as the tragic but inevitable outcome of the abandonment of these two young men by their reckless, selfish, and irresponsible parents.