If Thor holds back when he fights mortals he has a questionable perception of how durable people are supposed to be. Thor had no idea what Caps shield was, make no mistake: Thor flat out tried to murder Captain America in that instance and NOBODY said a thing. Look at the forest afterwards, that hit was meant to pulverize Cap. Thor was mortal before, he knows how frail they are, so I don't even think we can just say Thor is ignorant to how weak people are.
It's not surprising Thor tried to kill Cap, think about where he comes from and who he was raised by? His father has bi polar disorder. One movie he is all against war, the next movie he is the opposite. When you're raised by a bi polar god you're going to turn out wrong.