Is it possible to decrease your reputation without causing much trouble? Cause I still have a lot of quests that increase reputation to do, and I don't wanna lose my evil people.

Step 1: Go inside of someone's house (preferably an unimportant NPC, ex: "Peasant").

Step 2: Kill everyone inside the house

Step 3: Walk outside the house

Step 4: Initiate a happy conversation with the guard.

Your reputation should be at least 6 points lower after doing this. Or you could just kill one person.

Sure. Use the Slayer, if you've gotten that far. Two points off every time you do it.

Or, get caught stealing. Have someone who sucks at it try to shoplift. Like Nalia.

SPOILERS!
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Pick up Viconia and talk to the elf-hating dude in Waukeens promenade.
Killing him will cause the nearest guard to become hostile. Be sure to kill them a few screens away, or the nearest merchant will become hostile too.
If you haven't picked up Viconia, then it's a good idea to do so (government district) as her presence decreases your reputation by
In the graveyard district you can get a quest from a dude named Tirdir (he has been kidnapped and buried alive).
When you encounter the kidnappers, you can choose to ransom their victim yourself.
Worth -2 rep. and a pair of silver pantaloons.
Finally (I think) the silver dragon Adalon (in chapter 5) is worth -3 rep.
Her blood can also be used to enchant a suit of human flesh armour. I think that decreases your rep. as well.
The armour itself can be found in the bridge district in the "bridge district murderer" quest.
All these are programmed actions, so they won't cause any problems.

There are some acts who will lower you reputation very serious. In the Cooper Coronet there are some sleeping dwarfs. Kill one of them and you'll loose 10 points of rep.

Let sleeping dogs lie.

"Let sleeping dogs lie" >>>> I don't get it, what are you talking about? That sounds like some wierd statement you'd read from a fortune cookie or some wacko horoscope...

Those things will probably work. I migh as well try them out. I didn't get the "Let sleeping dogs lie" thing too.

The amount of rep you lose for killing an innocent civilian depends on what your rep was in the first place, not on the identity of the victim. If it's 3, I think you only lose one point for killing an innocent. If it's 20, you lose 10.

I think you lose rep points for beating on that Trax (sp?) bloke who confronts Jan rather than lying to him about your conversation, but Im not sure. Verification, anyone?

Yes, you do. That happened to me in my "Wally" game. You can look in that thread to see exactly how many points you lose; I don't remember off the top of my head.