Mindless undead will, if not controlled, seek out and destroy life. This is the result, I suspect, of positive and negative energies' tendency to attract and annihilate one another as is explosively demonstrated by the invariable occurrence upon a xeg-yi and a xag-ya, energons composed solely of those energies given sentience (described in MotP), encountering one another.
Now that is nice, I will have to remember that one.

Remember however that killing is only evil if death is seen as undesirable. As astonishing as it may be throughout much of western history people lived their lives in perpetration for the hereafter. If undeath was viewed as an essential part of that perpetration or as the afterlife itself then undeath and the killing it causes would not be perceived as inherently evil.

The creation of intelligent undead is, in most cases, worse still. Most intelligent undead have the need to feed on the living. This leaves that unfortunate soul with the choices of being tortured by his hunger, causing suffering to still living creatures, or killing; not to sustain his unnatural life, but simply to ease the hunger or maintain some ability. The person who visited this hell upon another creature is either committing torture every second that undead creature is animate or spreading evil in the world by creating a creature that will invariably do evil for the sake of easing its own pain.
For some people this would not present much of a problem. For people with all important long term goals it could be viewed as a necessary sacrifice. For people who either don't care or positively enjoy the suffering the inflict on others this would be no problem at all.