I was using Imoen to steal some items from a house (she is good to lockpick doors, drawers etc.) in Beregost, when a guard entered in the house, saying I was going to jail for my crimes. I killed him, and stole his armor (good armor, by the way) and weapon. On my way to Nashkel, another guard tried to arrest me, and I killed him, too. After that I arrived at Nashkel but every guard in the city attacked me, killing me and my 5 friends. There is a way to calm the guards? If I allow them to take me to jail, what happens? Game Over? The only solution is to load from a old save where I haven't invaded the house in Beregost?

Go to a temple and donate cash until your reputation is better than 5. For every reputation point under 6, the percent chance to have Flaming Fist spawn and converge on your party increases (from 10% chance at reputation 5, to 50% at 2 reputation, and 100% at reputation 1).

Donation Rates (from the Manual Appendix):
To go from Rep 1 to Rep 2: 500g
To go from Rep 2 to Rep 3: 400g
To go from Rep 3 to Rep 4: 300g
To go from Rep 4 to Rep 5: 200g
To go from Rep 5 to Rep 6: 200g

Note, you need to give each amount separately, because the church will only raise you 1 reputation per donation; whether you give 200g or 200,000g at reputation 5, you are still only going to increase to reputation 6.

I think I have reputation 3 ATM, if I donate money, the guards will leave me alone? Or they will still try to kill me? The guards trying to arrest me are weak, but the guards in Nashkel are destroying me easily.

Thanks for helping me.

EDIT: The Amn guards are in peace with me after I donated a lot of gold coins to a temple. Apparently, people with bad reputation are forbiden to enter in Nashkel.