Well, there are hundreds of thousands of dead people in Iraq that might disagree. The US invaded that country under cover of a series of blatant lies, perpetrated a large number of atrocities while there, and all in all made the country into a brutal mess.
No, the US is not Syria, not exactly. It does not, for the most part, imprison, torture and kill its own people. It does however bomb, kill and occasionally torture a fairly large number of people overall. In both cases the victims tend to be brown(er) people, but that hardly differentiates the two countries.
Assad deserves to go to prison forever, as do a large percentage of his henchmen. But so do Bush, Blair and (sadly) Obama. There is no correct amount of slaughter and torture, no threshold beyond which it becomes a crime against humanity. It is a crime (morally and usually legally) and they have all committed many.