So, there's a lot of issues here, both with your understanding of this history and your former professor's interpretation. I think a good way to tackle this is by focusing on just the violence aspect of the Klan from its founding to its collapse.

As a little bit of general background, the Klan was originally founded in 1865 in Pulaski, TN by former Confederate officers. The Reconstruction Era of the US (1865-1877) is a time of flux for the nation. No one was sure what would happen in those initial days following the end of the Civil War. Racial lines were redrawn across the nation as former slaves are made citizens and given the right to vote and hold office. The major cities of the South lay in ruins and roughly 2% of the nations population had been killed. Also, most of the Southern states didn't even have completely functioning governments at this point as they had not been allowed to rejoin the Union yet (hence the political purpose of Reconstruction). This is the environment that the Klan emerged from. The original purpose of the organization was that it was almost like a social group. In 1865, the war had just ended and no one was really sure how many rights freedmen were going to be given. It should be noted that the 15th Amendment (giving African Americans the right to vote) wasn't even passed until 1870. African Americans were already voting in some places before this, so the amendment is more to protect those voting rights from being rolled back in some places and expanded into others. All this to say that the Klan founded in 1865 isn't singularly focused on voting rights because at that point most white southerners can't vote either. It's as Reconstruction starts to pick up steam that the Klan we are familiar with begins to emerge.

By 1867, the Klan had begun to spread across the South and take on a more nefarious purpose that included violence and intimidation. It's also important to note that groups like the Klan were popping up all over the place in the South, the Klan just happened to be the most widespread. These groups weren't singularly focused on voting issues. Anything that threatened the hold of white supremacy could be targeted. That might include voting, but it also often included targeting economically successful freedmen, educators in the freedmen communities, religious organizations, white Republicans, etc. Anyone or anything that offered help or hope to the freedmen communities was a potential target. The Klan from 1867 to around 1870 can be seen as both a success and a failure. The success of the Klan was that it gained major notoriety and did cause terror in freedmen communities. They also helped reinvigorate a white Southern masculinity that had been badly bruised by losing the Civil War. However, because the organization was made up of dozens of local chapters with no overarching leadership, it was ineffective at regaining broad political power for Southern whites. That's not to say the Klan was "stupid and ineffective at dealing with the recently freed black slaves" or not taken "seriously as an outlet." In fact the threat of the Klan and other groups was taken so seriously that the US government passed the Enforcement Act of 1871, also known as the Ku Klux Klan Act. That law allowed the US government to arrest and imprison Klan members without trial and use the military to enforce civil rights for freedmen. In the wake of the government crackdown, Klan membership began to wane. However, that's not to say the Klan's tactics went away. They were actually perfected. By 1874, new paramilitary groups like the White League in Louisiana and the Red Shirts in Mississippi and the Carolinas were popping up. These groups were much more successful and were almost entirely focused on voting rights. These groups can basically be understood as a military arm of the Democratic Party. The culmination of this effort was the Mississippi Plan in 1875 which saw white Democrats in Mississippi take control of the state government for the first time since the Civil War. They accomplished this by intimidating Republican political candidates and freedmen voters, posting armed guards outside polling areas, openly parading through towns armed with rifles, and even massacring people at Republican political meetings. Unlike the Klan, these groups normalized their behavior by doing it in broad daylight and inviting press coverage of their actions. The "plan" was so successful it was repeated in virtually every Southern state over the following years. By 1877, Reconstruction had ended and blacks in the South would be on the downward slide towards Jim Crow.

After 1877, while the Klan might still have pockets, it was basically defunct until the 1920s. Other paramilitary white supremacist organizations existed and did a much better job than the Klan, so it just wasn't "needed" for lack of a better term. You did correctly identify the turning point of the modern Klan as The Birth of a Nation in 1915. This is basically seen as the "rebirth" of the Klan. The new Klan has a massive membership (4-5 million) that stretches across the nation, not simply in the South. And it also was just a complicated organization. It's concerns were not simply limited to African Americans like the original Klan. In addition to African Americans the new Klan was concerned about Catholics, Jews, and the exploding population of immigrants from Southern and Eastern Europe. It was also a progressive organization that pushed its members to a high moral code and supported prohibition. Yes, you read that correctly. The Ku Klux Klan considered itself a morally upstanding organization that threatened people illegally running alcohol and even participated in liquor busts with local authorities. However, it was not to last. By the mid-1920s membership began to collapse again and it generally went out of fashion until the Civil Rights Movements of the 1960s. Even then it wouldn't swell into the numbers it previously had (except in North Carolina, but that's a discussion for another time).

So, to sum up:

• The Klan doesn't have one specific purpose upon its founding.

• Voting does become a major issue soon after its founding. However, it's one of many issues (I also should have found a place for this earlier. The whole white robes thing is a product of the rebirth of the Klan. They didn't often do that sort of thing in the early days.)

• The Klan did terrorize freemen communities to the point that the federal government essentially outlawed it.

• But it wasn't a successful enough group to win back political control for white southerners. That is left to later organizations that take the tactics of the Klan and use them for expressly political purposes.

If you want to read a great book that covers the white supremacist political violence (including the Klan) during the Reconstruction era, I think the best one is But There Was No Peace: The Role of Violence in the Politics of Reconstruction by George C. Rable. University of Georgia Press I think.