Why Are the Undead Ones Always 'Evil'?

Why—long-&-short: because their agendas don't fit with the living-people's ideals.

I'm thinking of THE MUMMY. If you'll remember, the mummy was 'horrifically mummified' (in a mummification-process never used before or hence, whence the person was mummified-alive, buried in a casket full of flesh-eating insects, and placed beneath a curse of eternal-death that would kill all living people if broken) for defocating the Pharoah's bride. (True, it was with her permission; but she had been 'stolen from the Pharoah' by somebody!)

The mummy (Imhotep) was accidentally resurrected and just proceeded to do what he had sworn he would do: attempt to resurrect his love, The Queen (Ahnuksunamun). Swearing to do that, he doomed himself to it. At the end of THE MUMMY RETURNS, when Ahnuksunamun's reincarnation (who had sworn herself to Imhotep) wouldn't try to save Imhotep from the pit of Hell, Imhotep realized that his oath to her no longer held him, but that the penalty (which he had to incur to hold to that oath) did hold him.

What that story (and the sum of the other stories about the undead) tells us is that life goes on without you after you die; whether you like the effect you left on it or not, you'll only come back to find life going on without you (even though you're there).

You do "only live life once," but that's no excuse for "fun!" You 'live life once' FOREVER; if the founding-fathers, the educational scientists and the prolific playwrights had been more 'party-animals' than 'industry-leaders,' they would no longer matter to any of us. We could no longer hold up their lives as examples for our children!