One more thing that could differentiate evil and good:

evil will kill evil
good will protect good

A secondary goal for the evil character (besides stealing souls of Holy players) could be to destroy other evil characters to steal their power.

The situation this creates is that evil players CAN band together but will always have to look over their shoulder to make sure another evil character isn''t about to stab them in the back.

Good characters have the advantage of safety among other good characters (who can''t attack other good players, if they want to remain ''good'')

Little more on this:
<a href="http://www.gamedev.net/community/forums/topic.asp?topic_id=38079>Good and Evil: Flow of Power

Another idea-- Instead of making the evil creatures all monster types why not add some human elements. The whole day and night weakness thing would keep them from mingling during the day but evil humans would add fear to the holy ones who might always have to suspect the lone stranger who has entered the tavern by night. I was thinking of the Dark is Rising series in which good and evil (The Dark) clashed in everyday meetings, but sometimes only suspected something amiss. The evil ones know they are evil, but the good guys might not. This adds to plotting and traps to be set to capture the souls.

--"Come to my mansion for the party tonight, I want you to meet my friends. Don''t worry we will be inside and safe from the evil out there." he said with a fleeting smile that just barely revealed his crimson-stained teeth. Harold did not notice this, however, and gladly agreed to the invitation. He and his sister would attend, arriving just before sun down to insure safe passage. Of course the nobleman''s other guests were due to arrive shortly after ready to feed their hunger for souls.--

You could make them vampires if you want to stick with monsters. This just would add to the intrigue of the RPG. (if that is what this is) Never knowing a person''s alignment would be cool and scary. Players could then play satanic assassins or holy infiltrators pooring holy water on evil ceremonies in dark crypts.

--Take revolution seriously, but don''t take yourself seriously.--

Your idea as originally stated sounds more like an FPS with RPG elements than a full RPG(nothing wrong with either). The subsequent suggestions made it more RPG like in my mind. If you want to stick with a more FPS like feel(don''t care much about your character because there isn''t much advancement), you might want to consider what kind of advancement is available. Typically, the best fighters are the best at the game in an FPS, whereas the best fighters in an RPG are the ones who have the highest level, so increasing weapon prowess is incongruous. Increasing weapon power is always a possibility. Increasing things like stealth, or(as per Deux Ex), decreasing the amount of vibration in their shots is as well.(but this is prone to cheating) Firearms(HL mod with RPG elements) has things like Artillery skills that you could get, which increase your ability to assist the team but don''t really make you that much greater on your own.(it had lots of those, too)