other!Harry's soul - updated Harry's a wizard, but there's more. He's also a very broken little boy, tortured, molested, full of pain. And from this broken boy, emerged an angry, sad, lonely man. He's not intrinsically evil - he wasn't born that way, he was raised that way.
There is rage - at Justin, at what he did, at what he made Harry do; at Justin's other students who abandoned him because he learned to fend for himself; at the White Council for turning their backs on him and not stepping in when they knew what Justin was and what he was doing; at Elaine - (whom he knows, but never loved) - because she's helping the Council.
There is, buried deep, the secret, the side he uses, but doesn't show - a part of why the Council sent him - manipulation. He can manipulate people into doing things. That's how Justin was killed in the battle against the vampires. Harry manipulated Justin into working his way further into the battle - used Justin's arrogance to force him to do stupid things - and it got him killed. The Council could never prove that Harry had done anything, but Harry knows he did it.
And - the time he almost killed Michael (Gabriel) - only he wasn't actually there, he was working 'remotely' and controlling the demon; he almost killed Michael.
And the time he nearly killed Elaine, who was helping some wizards from the White Council hunt vampires. That resulted in the death of one wizard - not directly Harry's doing, which is why he wasn't executed. He was sent to Milliways instead.
There is also the tiny part (that grows larger with each passing day), buried way back, deep in the corner of his soul, that is beginning to care for Stephen and David and Adam and Bethany... maybe others. Harry would have - before - sooner admitted to having Justin killed than to having any kind of 'feelings' for anyone. Slowly, very very slowly, this is changing.