Unfortunately I have bad news for you, there is no way to get out of the Evil route once you start it. So you screwed up on this one and your only choice is to create a new character and go for the good ending next time (or if you have an export bak file somewhere you can try reloading that one and starting from that point again) You have to decide for yourself i you want to do the Evil ending with this character or refuse to commit such horrible acts. (You do get an achievement for it though, so that's pretty nice)
To make matters even worse, the Evil ending is (in my opinion) the hardest ending of all of them since between you and immortality are standing some foes that can rival the horror and this time you won't have any help with it.
You have to finish other quests before entering upper level at ark. You can finish sewer of horror before that, just get the armor and weapons from ark first. I also screwed it up once, so don't worry you're not alone
I see, then it's time to start from zero again, I recommend that the Wiki should have a warning at the first points of the detailed guide that should say something like: "WARNING: by talking to Valeran knowing the truth of your ancestors and telling him that you want to join the Magistrate and be immortal will start the path to Evil Ending and THERE WILL BE NO GOING BACK, if doing this you cannot perform the Bad Ending or the Good Ending and the only way to remedy this will be to reload the game from a Save before talking to Valeran."
Well, the good part of all this is now I know about my mistakes and I could do a better second game with a better build for a Rogue, thanks.