Bernie Sanders should certainly tell his supporters to vote for Clinton. He stated throughout the campaign that Clinton is a far better candidate than Trump and he would support her if she defeated him in the primary. Clinton and Sanders disagree on some issues, but another Democrat is far more appealing to Sanders' supporters.
Yes, I think that Bernie Sanders should tell his supporters to vote for Clinton instead of Trump because Trump is an extremely divisive presidential candidate and Clinton is more qualified than Trump. Although Bernie Sanders' supporters dislike Clinton, she is the lesser of two evils when matched up against Donald Trump.
Bernie Sanders should not tell his supporters whether to vote for Clinton or Trump, let the voters decide. With Secretary of State Hillary Clinton becoming the presumptive Democratic presidential nominee, it is obvious that supporters of Bernie Sanders would have to make a tough choice, between voting for a Republican candidate or voting for a democrat, Clinton, whose views stand in contrast to Sanders.
Bernie Sanders does not have to offer Hillary Clinton an endorsement, nor does he have to tell his voters to vote for her. Many believe that the Democratic Party's primary was stacked in favor of Clinton. Therefore, Sanders should feel no obligation to help Clinton with the election unless he wants to. In short, no, Sanders should not have to direct his supporters to vote for Clinton instead of Trump.