The "if men were angels" quote was meant to imply that not everyone has communal interests in mind and that certain government officials are inevitably going to push legislation that is in their own interests, rather than in the interests of their constituents. Madison emphasized that a system of checks and balances would prevent this from happening and he uses the quote to show that checks and balances are necessary because men are not necessarily all angels. This also ties back into the ideas of liberty and equal opportunity that Madison seems to be trying to emphasize through this Federalist paper.