if someone fails to pay their rent is that against the law?

It's a violation of contract law . . . which means the landlord as the right to sue them for unpaid rent and can obtain a Writ of Restitution from the court - which would allow law enforcement to physically remove them.

It's not "against the law", but it is a breach of contract between that person and their landlord, in which the tenant can be subject to civil court suit.

Yes. It's a breach of contract. Civil proceeding

It would be a civil matter for the landlord to take up. He/she would have the courts on their side since you entered in a contractual agreement. You are the lessee and he/she is the lessor.

I don't know if it's illegal but you can be sued.