Bernie Sanders is a threat to Hillary Clinton's campaign. He has been a gentleman and has been clear on the issues throughout his campaign. The constructive tone of the debates should be the prevailing tone of the democrat party. By penning the "Anti-Sanders" letter, Clinton has lowered the discourse. The tactic might very well backfire and alienate voters who expect more civil behavior.
Clinton released a letter with foreign policy experts denouncing Bernie Sanders' position on foreign policy. There does not seem to be much in terms of any kind of professionalism with this, especially since it was exposed that the experts have ties to contractors utilized by Clinton. Not only is it unprofessional and rather passive aggressive for her not to formulate her own opinion, but it ultimately backfired because she got a subjective opinion rather than formulating her own or going to neutral parties for an opinion. Something like this could be viewed as a political faux pas as Clinton was unable to act on her own, and had to get support of those on her payroll, and the public can't garner objective thoughts from something that is written by those paid to say certain things.
It was extremely rude and unprofessional for Clinton to write an "Anti-Sanders" letter. Not only has bashing or shaming a rival politician been shown to not work in the past, it further shows the threat Clinton feels from Sanders. The letter was extremely rude and just illustrates Clinton's fear and incompetence.
Presidential hopeful Clinton was not rude in allowing her campaign to release a letter highly critical of her opponent's foreign policy views and credentials. This type of letter is a standard campaign tactic in Presidential primaries. Indeed, it also helps show some of Clinton's "toughness". Whether the campaign's position on Bernie Sander's views on Iran are compelling remains another matter. But the letter itself is a legitimate campaign tactic.