I think it's REALLY unprofessional because, I mean, ask yourself this question: Would you want a president who threatens the life of another person? Even if it was a "joke", which it clearly doesn't look like it is, it is still something we all wouldn't want in our future president. Just because Hillary Clinton is his opponent, it doesn't mean he has the right to threaten her life. Our future president should be friendly to his opponent, even if his opponent didn't return the favor.
Besides being unprofessional, this comment is in really poor taste, especially considering all the other comments that have been made about Hillary Clinton. What is more concering is that Donald Trump likes the comment, which simply proves that he has poor judgement and little to no impulse control when speaking.
This type of joke about a presidential candidate is completely inapproriate and could quite easily be seen as an incitement to violence by some of the more unstable people that might support Trump's campaign. For Trump to like this type of sick humor shows an unprofessional attitude that makes him unsuitable to take on the role of President.
Many things that come out of Trump's mouth are unprofessional, including this comment about Clinton. Neither presidential candidate is great, but if you're going to hold the position, you need a high degree of tact and professional discretion, both of which Trump severely lacks. Joking about things of this magnitude is not professional at all and not something a presidential candidate should support.
Yes, this is incredibly unprofessional. In most workspaces, if someone were to suggest that a competitor for a position deserved to die by firing squad, that person would face disciplinary action. Mister Baldasaro's statement is inflammatory and childish. Disagreeing with the politics of another person is one thing. Threatening the life of another person is unacceptable.