Is converting ethical?

I travel alot in Latin America for work. If you look at my pic you can see I look like I am from there. Any way from my trips to Nicaragua, Ecuador, and Brazil. I always seem to encounter missionaries trying to convert the locals. I was offended when I saw a protestant try to convince a Native (Indian) in the rainforest of Brazil that he lead a sinful live and need to go to his his bible study group. I was appalled when they were giving food to poor children only if they would take their bibles in Ecuador. And in Nicaragua I was approached and told that in bad Spanish that I needed to find Christ thru them and that my believe in Christ was all wrong. (I was with my wife and she had a crucifix on her, I am an Atheist, she is actually a Protestant, but wears crucifix for fashion.) I actually told them in English that telling people that there culture is wrong is a crime and if I believed in god a sin. Is there a line that should not be crossed????? Or is nothing scared other than your believes. And damn the other culture that you find inferior?