I believe there's been good justification of it being malignant. As the winds of magic are bent to one's will so are they controlled in turn. Metal wizards become more rigid and metallic, fire conjurors become obsessed with flames and heat and necromancers begin to see life as a useless thing that's too complicated compared to the silent perfection of oblivion. When Gelt became corrupted by undeath he began to see the soldiers under his command as mere fodder.
I confess that I always found the idea that magic directly changes your personality to be odd and even rather limiting - if I play a wizard in WFRP, I want to be able to define my character's personality without having to work around the narrow stereotypes presented - but I think you're almost certainly right about necromancy.
It's important to remember what necromancy is. Necromancy is not defined by its final, outward effects, but by the combination of energies that produces those effects. Necromancy, fundamentally, is a blend of
shyish and
dhar. It was improvised by Nagash through combining the traditions of the Khemrian mortuary cult with the principles of dark magic as taught to him by shipwrecked Druchii. We all know that
shyish is not inherently corrupting, and there's nothing intrinsically evil about magic that manipulates energies associated with death; but we do know that
dhar is always corrupting.
Liber Tzeentch and
Realms of Sorcery discuss this at some length.
Dhar is corrupted or polluted magic; a sort of stagnant, highly toxic magical energy that you get by squishing all the winds together until they lose their focus. Use of
dhar is highly dangerous, as its toxicity both rots the flesh and corrupts the mind. (Over-use of
dhar is why Necrarchs, for instance, are usually both insane and rotting away.
Dhar is also why vampires need to drink blood: vampire 'biology', so to speak, is powered by dark magic, so they need to regularly get rid of 'used' blood, which is suffused with
dhar, and replaced it with untainted blood.) Druchii have to some degree learned how to use
dhar safely, with the aid of elven mental discipline, but even they cannot protect themselves from it completely, and it probably does influence the sadism and cruelty of Druchii culture.
As with Balthasar Gelt, then, it is entirely unsurprising that learning necromancy would have negative effects on one's psychology. Necromancy is a type of dark magic. Dark magic makes people go crazy.
This HoMM 5 video sums up the evils of undeath nicely, skip to 17:40.
A vivid example indeed, but HoMM necromancy works a little differently. HoMM V's aesthetics are very derivative of WHF, though, so I can see the strong connection. It really is tempting to compare Sandro and Nagash, isn't it?