If I "stepped into the shoes" paid the house off because the owner was behind and mortgage and taxes and sue him. Can I keep the house?

If you buy a mortgage and/or tax liens, you would acquire the rights of the curent mortgagee/lienor and would be able to foreclose on those debts, meaning that you could force the sale of the house to pay them.  If you wanted the house for yourself, you could work out an agreement with the homeowner or you could bid at the foreclosure sale, at which time you would have an advantage since you could bid up to the amounts owed you without having to lay out any actual cash.  For example, if you are owed $100,000 on hte mortgage, you could bid up to $100,000 (leavign aside any fees incurred in the foreclosure process) and could satisfy it by cancelling the mortgage debt, while anyone elswe would actually have to pay it off.