Generally speaking, I don't think that violent offenders should be eligible for commutation by the President. However, without being very familiar with the case, it seems that Oscar Lopez Rivera is not just another violent criminal. He has an important meaning to many people, and his commutation has brought joy to many people who don't even know him. I think this is just a different kind of case.
Yes, there are some people who shouldn't receive a commuted sentence, because the President just commutes sentences at the very end for political allies. Obama should not be able to give favors to his friends. At some point, society has already decided what justice is for a serious offender, and society should know justice will be served.
The president should have the power to commute all sentences, even ones for violent offenders. There may be extenuating circumstances, which the president can consider. While there will be controversial decisions, such as Obama commuting the sentence of Oscar Lopez Rivera, overall this should be in the president's hands, much like a governor can pardon state criminals.
Only in very rare and special circumstances should a violent criminal have their sentences commuted by the President or otherwise. If the criminal has served the majority of their sentence and has shown complete rehabilitation, then possibly. Also, the nature of the crime should be taken into account. However, commuting sentences in the early stages of one's sentence is not a good idea.