Finding an English word

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:

For 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)

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

Now do you know what word it was, please?

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

Thanks in advance.