They're both sad and dangerous. To a full grown adult, they were no risk, but they would engage in very inappropriate behavior with children if they got the chance. They viewed this behavior kind of like my dog views it when I catch her in the cat food. She knows she gets in trouble for it, but has no clue why.
Of course, just like any other group, some of them are just frikkin mean. I've also had some counselers tell me that low IQ boys and girls have a very high incidence of being sexually abused, so they often develop serious anger issues and act these out as adults. These people are truly dangerous.
In San Antonio recently, a developmentally disabled teen who was also a registered sex offender killed a nine year old neighbor.