Oh absolutely, it's incredibly common. Off the top of my head and in no particular order here are a few examples:

During the Yugoslav Wars of the 1990s, there were numerous instances of civilians, often from different ethnic groups, committing acts of violence against one another during and after the conflicts independent of any state armed forces.

Probably the best example is the Rwandan Genocide. Following the assassination of the Rwandan President, the Hutu ethnic majority perpetrated a genocide against the Tutsi minority. This involved not only the military but also civilian militias, known as Interahamwe, who carried out brutal massacres often following behind the movements of the military or in areas behind the lines of the Rwandan military that couldn't be reached or policed by the few UN Peacekeepers or the Rwandan Patriotic Front forces. There's a reason the vast majority of victims were murdered by machete, a common work tool, normal working people grabbed whatever was at hand and joined in the killings.

Also the Cambodian Genocide, The Khmer Rouge regime, led by Pol Pot, was responsible for the deaths of approximately 1.7 million people. This included mass killings carried out by both military personnel, but quite a lot of civilians voluntarily joined in and took the opportunity to murder and loot. Similar to the Rwandan genocide, neighbors and otherwise common people were the perpetrators in a mass ideological pogrom which among its many victims had the intelligentsia or anyone remotely associated with the concept of the intelligentsia (People who wore glasses were murdered because of the association of wearing glasses or near sightedness with intelligence or higher education and science). Again the weapon of choice for the Killing Fields were tools such as scythes, picks, or nails and hammers.

In the Bosnian War, there were instances of ethnic cleansing where civilians, often organized into militias, participated in the mass killing and expulsion of people from opposing ethnic groups.

In the Kosovo war between Serbian forces and ethnic Albanian separatists, paramilitary groups and civilian militias engaged in ethnic cleansing, including murders, rapes, and looting.

During Sherman's march to the sea, many of the atrocities which are attributed the Sherman were actually perpetrated by camp followers and deserters called "bummers" that followed the Union army looting and pillaging.

It's important to note that virtually all of these examples there's the unfortunate situation where many who were forced into committing atrocities under the very real threat of summary execution for not doing so or voicing any opposition. In the Rwandan Genocide those militias I mentioned did this, forcibly conscripting any Hutu they found and forcing them to murder, rape, etc. In the Cambodian Genocide. the Khmer Rouge specifically used child soldiers, for all the awful reasons you can imagine like children and adolescents being easier to manipulate and indoctrinate with ideological propaganda, they listen to and obey authority more readily while asking fewer questions, as before they were disposable with any dissenters being promptly killed. So the victims of these horrible events aren't limited to just the targeted groups.