So, picking up where I left off: when a person dies, they lose their spirit and roams around, normally decaying in a couple weeks, unless they have a strong unfilled desire that sustains them. Spirits are ethereal and thus can't interact physically except with other ethereal things.
There are tons of people dying every day, especially in a D&D world. But most of the dead have zero impact on the world of the living. Those that do, such as vengeful spirits, can attack other spirits, including the spirits of the living. They can basically harass people, rarely killing. The most dangerous spirits are those able to make psychic possession attacks, since a successful possession means they now have a body to use for their goals.
What does this mean for adventurers?
Adventurers kill lots of people, and surely many of the spirits of their victims long for vengeance, but that alone won't leave behind an angry spirit. Your typical bandit who picks a fight with the PCs basically gets what he always knew he'd get, someday. They same goes for things like goblins. A surprise ambush of not immediately hostile creatures might create some vengeful spirits, but they would linger in the area and eventually fade away. Figure if PCs return to the scene of the crime within a week or two, there might be something waiting: make a reaction roll, with a Very Hostile reaction meaning a vengeful spirit is present. The third or later visits get no such roll.
Slaughtering a townie in cold blood, even if the townie deserved it, might be more likely to create a roaming vengeful spirit. Make the reaction roll, but any Hostile reaction means the spirit seeks vengeance.
If a townie slain in cold blood by anyone had something important to do, the spirit comes back as vengeful on a Hostile or worse reaction, but comes back as non-hostile on a Very Good reaction. These would be remorseful spirits, guardian spirits, and spirits of warning. These may be of interest to PCs, even if the spirit isn't "out to get" them.
Disturbing ancient spirits is also a possibility. Usually, this is caused by defiling something. Make a reaction roll if PCs go treasure hunting in a still-active cemetery or desecrate a Lawful temple.
Spirits should be difficult to reason with. They are not quite the same as the person who died, more like a psychic shadow of one facet of their personality. They aren't even fully intelligent, just intelligent enough to be dangerous. Getting useful information out of them should be difficult, if not impossible. Speak With Dead or Contact Other Plane may basically "wake up" the soul of the departed to answer some questions that a mere spirit can't.
Of course, the undead aren't quite the same thing as a spirit. Undead are spirits and sometimes souls that have been bound to a dead body, or at least something vaguely physical, in the case of spectres. They are not as free as the truly dead, but as a consequence they are more dangerous, since they can attack physically. Most cannot make psychic attacks, the way a spirit can.