Well I understand that people need to die and while I'd rather let nature do the work, I do have an idea that should work. Get a smaller black candle and light it on one of the nights of a waning moon. Meditate and ground your negative energy into the person you want to harm rather than into the earth like you usually would. Visualize whatever you want inflicted on them and continue channeling your negative energy into them until the candle burns out. Once the flame extinguishes, think of that person's health and even their life extinguishing as though they are the flame.
Now while this is the only method I can think up that falls most in line with magic practice, I've also never had an incidence of people's hatred for one person causing them any noticable harm and in case you aren't at all acquainted with the astral plane, then this may very well not work at all. Remember, if the universe decides to keep them well and alive and you yourself understand very little of it then your intention won't materialize. You could call this a failsafe against any unnecessary tampering.
Otherwise like I said, I'm sure you have reasons and I'm not against people dying. If they're despicable enough then dying could be an efficient way for them to... return to the light?