Andrea Harris recently wrote: "When I rule the world, this person will have his entrails fed to dogs while he still lives!" As threats go, this is not bad, but there's nothing like the classics for invective. Here's Catullus 108:

Si, Comini, populi arbitrio tua cana senectus
   spurcata impuris moribus intereat,
non equidem dubito quin primum inimica bonorum
   lingua execta avido sit data volturio,
effossos oculos voret atro gutture corvus,
   intestina canes, cetera membra lupi.

Here's a rather archaic but literal English version by George P. Goold:

If, Cominius, your gray old age, soiled as it is by an impure life, should be brought to an end by the choice of the people, I for my part do not doubt that first of all your tongue, the enemy of all good people, would be cut out and quickly given to the greedy vulture, your eyes torn out and swallowed down the raven's black throat, while the dogs would devour your bowels, the rest of your members the wolves.