Can you hold your moral high ground even if you over-bill your flatmate?

Your flatmate doesn’t help with any of the chores at home and you have to do everything from getting the groceries to running around paying bills.

Your flatmate doesn’t help with any of the chores at home and you have to do everything from getting the groceries to running around paying bills. On top of that, he is always late with settling the accounts. Tired of the nonchalance, you start over-billing your flatmate for the shared expenses. Would you call it cheating?