The United Nations has drawn up a list of individuals and military units it believes are guilty of war crimes in Syria, though Human Rights Watch also says that opposition forces are guilty of torture and summary executions as well. HRW spoke to former prisoners captured by the Free Syrian Army who claimed they had been beaten and mistreated, and some FSA fighters admitted that forces had executed some Syria soldiers and police without trial.
The U.N. Commission of Inquiry on Syria says that it has been gathering evidence of human rights abuses and recommended that the entire situation be referred to the International Criminal Court. They also claim abuses on both sides, but say the actions of the Assad regime are much more widespread and systematic. Human Rights Watch has previously published a report accusing Assad of running a network of torture centers.
One thing that both sides agree on is that Islamist fighters have become a growing part of the fight. The commission says there's an "increasing and alarming presence" of fighters, some of them working with the opposition and others operating on their own to spread their cause. The Syrian government has always maintained that foreign fighters (backed by Western powers) are responsible for the unrest, but there's no doubt now that terrorists are playing a role in the war and no one is playing by "the rules."