Suppose you say something really really provocative like "The imposition of Shariah law in the United States would be a violation of the First Amendment and is completely unacceptable." Then as a result of that, at least in part, some imam who is also a chaplain on a military base gets really really pissed off and goes on a shooting rampage killing several officers and enlisted men before being shot by police officers. Among his personal effects there is an iPod with an MP3 file of your comment that he had obviously listened to. To what extent are you willing to be held responsible for the actions of this crazy imam that were "at least in part" because of the statement that he heard you make.
In this case, the two roommates knowingly committed a crime against this kid. The crime indirectly resulted in his death. There may have been other circumstances involved, but a direct connection between Crime A and Death B can be shown.
It is about personal responsibility- they may not have intended the results, but they chose to commit a crime that resulted in the kids death.
If a guy robs a liquor store and the store owner, so in fear for his life, dies of a heart attack, the robber is charged in his death. No, he didn't directly kill the guy, and there may have been other circumstances that contributed to his heart attack, however, Crime A can be directly associated with Death B.
Look at the recent case in Florida, did anybody express their anger at the Muslims that were threatening violence over the Koran burning? No, it was all reserved for the guy burning the Korans.