The goblins are created by the land and are tasked to defend it from imbalance. They are sustained by the land and do not need to eat. They are not born and thus do not naturally die. Their methods are the ones proven most effective, warfare and pillage. Whenever a certain faction becomes too powerful and threatens the land with their undue influence, the goblins rise up and do their duty. Over the years goblins have learned that dwarves are especially prone to excess so they tend to send raiding parties, even against small settlements, to keep them in check. Preventative-maintenance, if you like. If a dwarven civilization manages to grow to unhealthy proportions despite the goblin husbandry, with their forges consuming forests, their butchers decimating species and their burrows riddling the earth, the goblins must cull them for the greater good. The dwarves have no concept of the greater good and so ascribe notions of evil to the goblins.
Goblins adopt members of other species for investigative reasons. They hope to learn more about the most troublesome species and perhaps find a way to influence them to take on a less destructive path through life and thus avoid the goblins taking iron to them like gardeners to weeds. With the latest update the goblins are probably investigating the badger menace, those bastards are everywhere.