A big element of their case that's oddly not mentioned in this Guardian article is that the violence was foreseeable and nothing was done to stop it. The tensions did not bubble up overnight, and this was not the first instance of violence in an otherwise peaceful place. There were many signs that something like this could happen, and Unilever declined to spend resources on keeping the workers safe. As a result many of the workers were raped and murdered. Now the survivors are seeking justice, and they deserve it.
Unilever are the corporation who was ultimately profiting from the employees' work, so the survivors have a right to seek justice from them directly.
Citation needed - I never heard of a corporation being required (or allowed) to do law enforcement (private security companies might be contracted bt the government, but simply going and doing is most definitely not possible). The UN declares that a responsibility of governments, and one of the keys to the land claim and recognition of the state.
In some jurisdictions it's legal to stop a crime by appropriate means, but it is not a responsibility of any private person or entity, and it also does not mean you do it normally - that's where you're crossing into the "private army" territory.
What ever happened to prosecuting perpetrators instead of bystanders? What ever happened to the responsibility of government to prevent crimes?