I'm looking for an English word I knew beforehand but now I've forgotten!

The root of the word pertains to the word "cause". It's also a noun that refers to "persons".

Let's look upon it, as an example:

You tell a someone: You're the causer of all of these problems. ("causer" is not the word I'm looking for, but close to it and with the same meaning)

Actually, we use that noun to say the reason for the problems is that person solely, and he is guilty/responsible for all of them.

The synonym names close to it are: Cause, responsible, reason and son on.
Now do you know what word it was, please?

If you mention it, I'll quickly recall it.