Can you explain this? I have met Mormon people and they seem very friendly and kind, but do they believe that there is a connection between dark skin and bad behavior?

It is NOT a lie. spencer W. Kimball, as recently as the 1960's, suggested thaat white Mormons donate blood so that they could give transfusions to Native American children to "lighten their skin". The Lamanites were (according to the BoM) punished with darened skin, and Mormon bishops teach that black skin is the mark of Cain (that is precisely what I was taught when I was raised in the church). Until the church was threaatened with a lawsuit in the late 1970's, blacks were not eligible to hold the priesthood in the LDS church.