You can't kill somebody with aspirin. Trust me.
You can cut a body up and flush it down the toilet, it's been done in prison, and that's a pretty small toilet. The skull is the biggest problem, you'll have to crush it into pieces. Dismember in the bathtub, cleanup is simple.
However, if your plumbing system can't handle the relatively heavy workload, it will clog up the pipes. Plumbing repair will get you caught, that's what happened to Dennis Nilsen. So know your pipes before you try that.