When someone running for the highest office in the United States has no political experience and many failed business attempts, it should be clear that they are not exactly fit for the presidency. Time and time again, Trump goes on record to contradict himself in an effort to appease whatever person he's speaking to - he's always been a loose cannon, this is not a new thing, and Republicans should have realized that a long time ago.
Republicans should have known Donald Trump was a loose cannon. He has been making bold and controversial statements for decades. Over the last year, his statements have only become more controversial. Those that thought he would contain himself once he gained the nomination were not paying enough attention to his track record.
They should have, and I'm sure they did, but the great thing about this nation is how few actual qualifications someone need to have in order to run for president. There are not limits on who can run, besides age and citizenship, and for a political party to go against this would break that law.
Most Republicans should have known that Donald Trump was a loose cannon. Republican primary voters should have known the risks of nominating the bombastic businessman, because his offensive remarks were likely to offend much of the electorate. Therefore, Republicans only have themselves to blame for nominating Trump if his remarks end up costing the party the White House.