You could not have an oral agreement to rent for 6 months since that is against the RLTA.
RCW 59.18.200 (Tenancy from month to month or for rental period — Termination — Armed Forces exception — Exclusion of children — Conversion to condominium — Notice) provides: "(1)(a) When premises are rented for an indefinite time, with monthly or other periodic rent reserved, such tenancy shall be construed to be a tenancy from month to month, or from period to period on which rent is payable, and shall be terminated by written notice of twenty days or more, preceding the end of any of the months or periods of tenancy, given by either party to the other."
Your former landlord likely is not going to get any court to order you to pay rent for the 6 months you did not occupy the premises. In a contract, each party has a duty to mitigate damages resulting from the breach of the other party.
Given how good the rental market is for landlords now, it is doubtful that your landlord cannot get a new tenant into your unit in more than a month.
The lesson for you here likely is to read carefully all documents before you sign them. If you want something changed on the document, have the other party make the change before you sign the document.
"they did not give me back my deposit or a letter that state the reason". You can sue the landlord, perhaps for even twice the amount of the deposit, for the landlord not complying with RCW 59.18.280 (Moneys paid as deposit or security for performance by tenant — Statement and notice of basis for retention — Remedies for landlord's failure to make refund).
You should review the specific facts with your attorney to find out your legal options.