I've decided to run an evil campaign, and I just wanted to know if someone could give me a list of locations where the cursed items are in the game, a gift to the good guys :D

I know in chapter 6, Ribald sells a cursed plate mail in his secret stash that takes away a bunch of charisma and constitution. It's name starts with a 'T,' but I can't remember it off the top of my head.
Helm of Opposite Alignment: No idea. I don't know you'd want this one.
Girdle of Sex Change: Same as above ;) .

For some reason, the vast majority of Cursed items are found in the Underdark, which is strange since most of them could have been carried with the party in the transition from BG1. Whether they came with you or somebody else is open for debate, but one must postulate some unfortunate person(s) who got stuck with most of the Cursed items in the Sword Coast, went mad as a result, and crawled, Gollum-like, into the Great Rift.

Cursed Sword of Berserking -- Smaeluv Orcslicer in the Den of 7 Vales (Also in BG1)
Ring of Folly -- Svirfneblin Settlement (Also in BG1)
Backbiter Spear -- Riti in the Imprisonment Crystal (Dates from BG1, but not legally obtainable)
Bracers of Binding -- in a "window" in Ust Natha (Also in BG1)
Ring of Clumsiness -- Deirex's Tower (Also in BG1)
Trahcie's Plate -- sold by Ribald in Chapter 6

There are, of course, also a large number of Cursed scrolls and Potions, and the reward recieved from Pontifex for completing the Ritual quest is Cursed as well.

Not really that strange. Let's say that your party from Bg1 did happen to have most if not all of the cursed items in your inventory (and I say this without the use of the mod to keep your weapons from Bg1). Going over to Bg2, your equipment disappears at the beginning of it, and with Irenicus being associated with the drow in the Underdark, I figured it would have made sense they would have ended up there.

We aren't actually told if Irenicus was working with the Drow at that time. It's likely that he instead sold the items when he got them. I doubt he held on to them all the way till chapter 5 ;) .

Irenicus has elven mentality. Elves make elaborate plans that take decades to come to fruition. It is likely that he was in touch with the drow before you left Candlekeep, and he was waiting for just the right moment.
And nothing cements an alliance like gifts. And you know how the drow sense of humor works.

Well, that's true, we aren't actually told that, but there are those hints in the game that point out that he did. For example, take the gem of Equalizer being in a chest in his dungeon. It's origin (and description) say it came from the underdark and I don't think he would just happen to have it without being involved with the drow first. I'm not saying that the gem happened to be there absolutely means he did have an alliance with them first, I'm just saying it's a strong possibility.

And look at it this way. If surface elves anger you, and you need firepower at your back, who're the first people you'd go to for support? What's the first race that's just itching for an excuse to attack the elves?
"Orcs."
"No."
"Illithid."
"No."
"Humans."
"No."
"Dwarves? Gray dwarves?"
"No! Getting closer, though."
"::smacks forehead:: Of course! Beholders!"

The reward is very powerful... I personally haven't experienced it, but the effect only triggers rarely, and when it does trigger, it isn't like it's the death of the world. In exchange, you get a some resistance/immunities. Well worth it.

<span style="color: lightblue">Actually, it can be bad for, say, a solo Wizard... it can make one of the most important uses of a high level spell quite redundant... which could be annoying.