So the doctor for was a ....

No.

In a word: yes. He was in fact responsible for everything sinister that happened.

He, or in essence the little boy he used to be, was the negative force in the house. He just probably wasn’t completely aware of it consciously. If you recall there's a conversation around the middle of the film where Dr. Faraday is discussing the situation with a colleague. They broach the subject of studies that suggest poltergeist activity is not in fact ghosts, but the subconscious will of a living individual’s unmitigated desire (my words-I’m paraphrasing, but this is the gist of it).

Understanding the movie hinges on this information.

Recall the various connections depicted throughout the film of Faraday as a boy coveting the opulence and prestige of the house, wanting to be like those who lived within it, to the point of breaking off the acorn. Toward the end of the film this is visually presented as a connection to his present-day desire, which we learn at the very end was an obsession that he'd harbored all those years.

Remember even how he was looking at the girl playing with the dog early in the film before it acted very out of character and attacked her? Faraday’s subconsciously-generated force of will made that happen, manipulating events that let him worm his way into the household he so coveted.

But then at the very end of the film, he's had to dispel or kill everyone in the house in order to take up residence, and so he's shown as a sad little boy with a tear running down his cheek, finally having attained the object of his obsessive desire, but having to get rid of everyone else in the house to do so. He now has what he wanted, but it’s now lifeless and devoid of what had made it special.

Also, just found this interview off a quick Google and it seems to verify my own interpretation:

https://www.thrillist.com/entertainment/nation/the-little-stranger-movie-ending-explained