I think most criminals will prefer privacy coins like XMR rather than going in the hassle of bitcoin mixing service. But again the service of bitcoin mixer is not made to use it negatively. Its just that most people use it for illegal purpose and not for the good cause.
That doesn't count as an excuse that mixers can still be used for such malicious purposes. No matter how the developer or creator envisioned their mixer, it will still boil down to the fact that mixers can clean coins that are tainted due to illegal activities.