For those who do, what's your objection?
I have no objection.
Isn't traumatic or abusive upbringing due to actions in opposition to God?
Why would that not indicate demonic influence?
It's a possibility, but I wouldn't automatically discount other possibilities. Although, as I mentioned earlier, many human traits came about through the harsh struggles our ancestors had to endure just to be able to survive. Humans have had to work within the harsh realities and parameters of nature. How can anything humans do to survive be considered "in opposition to God" if we're assuming that God created these harsh realities and parameters of nature? Did the demons create the universe, or did God create the universe?
Why is that not working out?
Human nature. It's how we evolved, even if some of us try to do better. If God created us, then that's how we were created.
It is working out, though - at least to some degree. If we cite evil behavior as evidence of demons, there's probably less of it now than there was during the time of the World Wars. Or back when there was slavery, colonialism, and other aggressive, violent, malevolent activities. One could argue that demonic activity, if we agree that such a thing exists, has actually come down over the past centuries.