[Bombadil]

Perhaps what defines what creatures are truly evil by nature and which ones are not could further be explored by observing their aggressiveness. What I mean is the Balrog doesn't stir from Moria, but when the fellowship disturbed it, it attacked them. So for every creature:

The Black Riders - Aggresive, leaving Minas Morgul to find the ring by any means.
Old Man Willow- Well since he can't physically move you could call him aggressive for going out of his way to mess with the hobbits.
The Barrow Wights- The Hobbits came into their land, so once again cold be disputed, but they aggressively took the party captive.
The Watcher in the Water- Minding his own business was disturbed by Boromir throwing a rock i his pool.
Moria orcs- Fellowship entered their domain.
The Balrog- As I said above, they disturbed it
Gollum- Aggressive, going out of his way to re take the ring.
lots and lots of orcs (and Uruks)- Aggressive, in terms of Parth galen and Helm's Deep.
The Fell Beasts- I suppose you would call them aggressive, as they bore the ringwraiths.
Shelob- Was disturbed in her domain
The ghosts of the unfaithful- disturbed
The Mouth of Sauron - at black gate he was called forth, but Lieutenant of aggressive army
Sauron- Very Aggressive

Now at this point you're probably wondering what my point is, after reaidng all those "aggressives." When an Orc strolls into Rohan, it's killed. When the group of (Aggressive) Uruks ran through the Ridder-Mark with Merry and Pippin, they were on the land of the horse people, and they killed the orcs for that reason. The Rohirrim were even hostile towards Legolas Gimli and Aragorn. So why label creatures as evil if they are only defending themselves from unknown creatures, or ones known to be their enemies in the past? My point may be a little far fetched but I'll throw it out there.