what does security deposit cover

[Roy Mitle]

I am a landlord. I have tenants who are leaving the house in a bad shape - the carpet is all dirty, need to clean the showers, re-touchup the paint etc. they are also leaving the couch and refusing to taking it with them.

I told them their security deposit will be used for this at which point they got mad.

I am worried they may do damage to the place on the last day (eg clog the drains or I don't know what all) and I can't do anything. 

I believe I am right - the security deposit is precisely for this. But its kind of grey area. they may say you don't need to re-touch the paint or clean the shower etc. 

Have other people had this situation - where tenants got mad on giving them only portion of security deposit back...