We entrust our elected leaders with an incredible amount of authority over our lives, and that authority can be terribly destructive if it is misused. This is why a candidate's temperament matters. The more powerful an elected official's office possesses, the more important it is to elect someone with a good temperament.
For example, when a candidate for office is asked a completely fair question and then flies off the handle, encouraging his followers to viciously personally attack the journalist who asked that question, it raises serious questions about his ability to handle the stresses of the office he seeks. When that candidate has another meltdown months later over that same question, it raises even more doubts about whether that person has the temperament necessary to serve.
This is especially important for an office that has authority in the realm of criminal justice. A malicious prosecution of a political enemy can ruin lives, even if the target of that malicious prosecution is completely exonerated. Investigations by regulatory agencies can destroy a business, as can foot-dragging on necessary permits and approvals for tax benefits. Whether it is the mayor of a small city or the President of these United States, someone with a bad temperament can do a tremendous amount of damage.
This is also why "voting with your middle finger" is a foolish, self-destructive and unpatriotic answer to the problems faced by your nation, state or city. If our country is to thrive, we need to elect the people with the best ideas, an even-handed temperament, strong personal morals and the necessary experience to hold office. Voters who choose someone because they are angry are likely to choose an angry candidate. Anger is not a substitute for substantive ideas and a strong personal character. We need to do better than we have done.