Hello, I killed some Orcs and looted them and then they simply got back up after a little bit and acted like nothing happened? What is going on?

The orcs of G3 are not the dumb monsters from G1/G2 anymore, instead being full-fledged NPCs. For the majority of them (except the ones in Nordmar or the occasional hostile party) you have to perform the finishing blow to kill them, just like with most humans. Otherwise you just knock them out for a while.

I did not know there was a finishing blow. I feel somewhat silly now XD

You target the downed enemy, then hold the right mouse button and klick the left one for some seconds, then the hero will stab the downed enemy :)