Why does Salvador Allende have such a negative reputation in the US and why was he killed?

Here are some questions I have answered that relate to this topic.

How deeply was the US involved in Pinochet's coupe in Chile in 1973?
Is there a theme for the motives behind America's foreign coups?
What kind of relationship was present among Chile and Cuba when President Salvador Allende was in power?
Why do people hate Henry Kissinger? Is there justification to call him a war criminal?
See my user profile page for more answers on Latin America and other topics.

THE BACKGROUND:

Allende always seemed like a fairly positive person for the nation.

I don't know if you can make a statement as to whether or not Allende was good for "Chile" as a whole, given socialism's mission of raising up the lower and working classes and destroying the owning class. Allende enacted economic policies that did benefit large sectors of Chilean society, notably in the fields of labor rights, social programs, and land reform, together with those, such as industrial nationalizations, which clearly hurt domestic and foreign capital owners. It can seem easy to point out the winners and losers by drawing class lines, but for many, whether or not their quality of life improved or declined had to do with both their perception of what was important to them in their society and the economic conditions that may have arisen more out of a reaction to Allende's policies than to the policies themselves.

Salvador Allende was elected president in 1970 with 36.6% of the vote in a field of three candidates. He was the candidate of the Socialist Party and the leftist Popular Unity coalition. As he was not elected with a majority of the vote, he was confirmed by the National Congress. The CIA and private corporate interests joined right-wing elements in the Congress in campaigning against the confirmation, in addition to employing economic pressure and an abortive military operation which the commander in chief of the army died stopping. After the confirmation, CIA agents posing as U.S. military officers tried to pressure the Chilean armed forces into staging an anti-Allende coup by threat of withholding military aid. For the next three years, the CIA, State Department, and multinational corporate actors such as Anaconda, Kennecott, and the ITT Corporation conspired with the Chilean right in efforts to destabilize the economy and incite unrest against Allende. In the words of Richard Nixon, they wanted to "make the economy scream." As for foreign support in favor of Allende, former KGB agents have claimed that Allende received money from the U.S.S.R., but his relationship with the Soviets was relatively minor compared with his relationship with Cuba. Though the traditional narrative places the Chilean coup in the international scope of the conflict between the United States and Soviet Union, it was more connected to the American conflict between the United States and Cuba.

Allende's term was marked by the nationalization of key industries, most notably the copper mines (which had previously been partially nationalized). Some land was nationalized, to either be operated by the government or distributed to dispossessed peasants. This was accompanied by protracted economic crisis, as foreign lending was cut and the opposition led strikes to subvert Allende, backed by the U.S. and the ITT Corporation. How much of the economic crisis was caused by Allende's policies and how much by the response of the right-wing is a contentious issue and I don't have enough economics training to be comfortable speaking to that.

In March 1973, congressional elections were held in Chile. The Christian Democrats and the National Party, which had separately run candidates in the 1970 presidential election, formed the Coalition for Democracy against the Popular Unity coalition.

The National Party, which had won 35.27% of the presidential vote in 1970, won 21.7% of the vote for the Chamber of Deputies (34/150 seats). The Christian Democrats, which had won 28.11% of the presidential vote in 1970, won 29.2% of the vote for the Chamber of Deputies (50/150 seats). Other members of the opposition won 5.1% of the Chamber of Deputies vote, with 2 seats for the center-right Radical Democracy Party and 1 seat for the center-left Radical Left Party. Overall, the opposition coalition won 56% of the vote for the Chamber of Deputies and 87/150 seats, as well as 14 of the 25 contested Senate seats that year.

Meanwhile, the Popular Unity coalition, which had won 36.62% of the presidential vote in 1970, won 43.7% of the vote for the Chamber of Deputies with 63/150 seats. That was 18.6% and 28/150 seats going to the Socialist Party, 16.2% and 24/150 seats going to the Communist Party, 3.6% and 5/150 seats going to the Radical Party, and the remainder of the coalition winning 5.3% of the CD vote and 2 seats each going to the Popular Unitary Action Movement, the Christian Left Party, and the Independent People's Action Party. The UP won 11/25 contested Senate seats.

The UP was still a minority government, but it had increased its share in the popular vote by an unprecedented amount (with a mostly symbolic net gain of 2 deputies). To quote a secret CIA memorandum immediately following the election, "It is true that the UP may well win legally in 1976, and that even the prospect of a PDC [Christian Democratic Party] victory in the 1976 elections leaves much to be desired. The Chilean situation may thus be considered desperate..."

THE MOTIVATION OF THE UNITED STATES:

Why is he seen in such negative light in America? Is it just the "socialism" or is there more going on?

I can't tell you anything about the public perception of Allende in the United States, but the U.S. government certainly didn't like him. When you ask "is it just the 'socialism'," the answer could be "yes" for different reasons. The dominant narrative surrounding the coup has typically been focused on the Cold War between the United States and the Soviet Union. Allende is a socialist, pro-Soviet, and a clear target for the United States trying to stop the U.S.S.R. from gaining influence in the Americas. However, U.S. interest in Latin America runs much deeper and dominant Cold War narratives make things too neat and northern-centric.

I would suggest you read the links at the top of this post for more information, but basically, the analysis that has been emerging more recently (though it isn't new so much as more in favor) places the conflict within the context of a very American conflict. Tanya Harmer calls this the "inter-American Cold War," polarized between Havana and Washington. As to why we shouldn't simply view Cuba as an analog for the Soviet Union, see my answer to "Why did Cuba go with the Soviet Union?"

The U.S. economy relied substantially on extracting labor and resources cheaply and consistently from the region. When this economic dominance was threatened in 1954 by land reform in Guatemala, the U.S. overthrew a democratic government. The Cold War was used as a justification, but the conflict was driven by the United Fruit Company and its U.S. government ties. Following the Cuban Revolution, Cuba was bombed and attacked before relations between Cuba and the U.S.S.R. even warmed. Similarly, it was hard economic reality, masked by the Cold War background, that precipitated the U.S. coup in Chile. For the United States, the immediate economic losses to business in Chile were unacceptable and the region potentially taking inspiration from a democratic socialist government was threatening.

THE COUP:

Also why did Augusto Pinochet stage his military coup and have Allende killed?

Pinochet was not the undisputed leader at the onset of the coup. Following a Supreme Court proclamation in which justices appointed by previous capitalist governments denounced Allende for failing to execute court orders, the deputies of the National Party and Christian Democrats formally accused President Allende of "ruling by decree" and not disarming socialist militias, asking the military to step in. Allende responded, calling for a popular vote to resolve the crisis of government. Less than three weeks later, on 11 September 1973, the Chilean military bombed the presidential palace. President Allende gave his final, defiant speech over the radio and died during the coup. The official narrative given by the military was that Allende had shot himself rather than be captured. The Chilean state recently had an investigation that confirmed this account, though other medical examiners have concluded that his wounds were more consistent with assassination.

The Christian Democrats seemed to have thought that power would be handed to them after the elected president was deposed. However, the military junta, formed by the leaders of the Army, Navy, Air Force, and National Police, took power themselves. Pinochet, the commander of the army, eventually emerged as the head of the junta.

In the immediate aftermath of the coup, socialist supporters were rounded up, tortured, and murdered, most famously including Víctor Jara. The United States began supporting the new government and acting as an intermediary to resolve issues between corporate actors and the junta.

If I had to suggest any book on the coup, it would be Allende’s Chile & the Inter-American Cold War by Tanya Harmer.

Hope this helped.