In early conversations with Anders in DA2, he describes his link with Justice as a "frenzy" that comes over him at times and that he sometimes finds out much later what he may have done. His body and mind are partially controlled by a spirit, and this influence increases the longer we know him. He is not human, he is an abomination, pure and simple. The only difference between Anders and a demonic abomination is that Justice started off non-hostile and curious. Over time he was corrupted by his experiences with the mortal realm and Anders' negative mindset. Thus Justice becomes more and more violent and controlling, to the point of nearly killing a young mage whose only crime was attempting to surrender to Ser Alrik. Anders can't even stop himself from doing it on his own, Hawke has to intervene or the girl dies.
Except the line where he says he forgets things ONLY happens in rivalry path so it remains a plot hole which path is canon. In friendship route Anders glows on much lesser occasions (aka controlled by Justice) and the codex specifically mentions Anders
ONLY loses control when fighting Templars, otherwise thanks to Hawke's help as a friend (or lover) he has gained control over the spirit within. Its also harder to kill him on this path because you are not killing a psychopath who has lost himself, you are killing a person with different opinions. So what you said is wrong. People lose control when they are angry too, its just worse in Anders' case.