I have a fair number of headscratchers about this game..

[rji]

Also WHY did he kill the girl? He had more than enough strength to easily subdue the kid.

Because he's an evil and selfish prick

Spoiler below!

that didn't want to surrender to the shadow or let Alexander have all the 'power' from the warding ceremony. It's hinted that Alexander was afraid of him by that point, so he was enjoying killing the unsuspecting townsfolk.
Of course it all worked out in the end, his murderous appetite was satiated with one final murder.