I am a Scarecrow and the Demon Lord of Terror!恐怖复苏：从稻草人到恐惧魔神！

Ah it's not specifically stated in the novel, well the deriving pleasure from terrorising people is, but the biological underpinnings for your behaviour isn't that's just science. Of course you can say what sense does science make in fiction, but it is a fact that if you were to take all your memories and stuff them in a completely different body your behaviour would change and the more different the body the larger the change. In this case I am saying transferring memories instead of soul as in effect this is what happens in all reincarnation/tranmigration novels where the MC takes over a new body. For example say you could transfer all of your memories from birth to the present and overwrite them over all memories of say a chimpanzee which are still quite close to us on the evolutionary tree, now we will assume through fictional magic that the brain can actually manage such memories despite the differences in brains as otherwise we have no story at all and you just end up with a braindead monkey, you would still be far more aggressive and less in control of your emotions and desires due to the different brain structure and very different levels of hormones affecting you. Now take it a step further and force your memories in things like demons or demonic scarecrows that are born for doing horrible things and you can imagine the effect being far stronger when the body doesn't even have a semblance to a regular human.

Not saying the author had all this in mind, but it is a very viable reason why someone transferred into a body that is completely configured for terror and/or murder could very quickly spiral down this path even if they have their prior memories. Not saying it makes the character some kind of good guy, but that it actually makes a lot more sense that most if not all going through such a process would end up as an absolute psychopath eventually and that it is more a matter of how long it takes until they get there rather than whether or not they will get there.