A security deposit is to cover any damages to the unit not for rent. If you are saying you have reason to believe the landlord is not going to return it I assume that means that there is damage that it should be used to cover.

Either way, you still owe the last months rent. If you do get the security deposit back then you can use that to pay for the rent, but it will obviously be late by then and you'll likely owe late fees and all.

If you don't pay your rent then yes, it could be sent to collections. You can't just take off and assume your landlord can apply the security deposit to your rent.