Thor knew the tremendous and deadly force generated by pounding Mjölnir into something. It sure looks like he was bringing the hammer down on Cap with 'bad intentions'. He has killed many Frost Giants with it and who knows how many other foes have fallen to his onslaught. He even used it to destroy the Rainbow Bridge in the movie 'Thor'.
There's no indication that he knew about the energy/force dissipating quality of Cap's shield or its ability to survive such a powerful strike. He hit the shield hard enough that when the shield diverted the energy away from Cap, all the trees around them were leveled by the 'shockwave'.
Did Thor somehow pickup on Cap or the shield's abilities when Cap bounced his shield off of him and nimbly jumped to the ground from 20' up and was just going to show him who's boss, or was Thor intending to remove this new threat from the gene pool?