I don't actually think that libertarians are evil, but I think they're naive and idealistic and I resent them because they want to enable the evil forces of the world to have their way and exploit everybody. So the proponents may not be evil, but the effect would be evil and disastrous.
Libertarians, as I understand it, basically want unchecked capitalism where corporations do almost everything and there are few to no regulations put on them.
But the modus operandi of a corporation is to do anything they can to increase the bottom line, even to the detriment of consumers, employees, the community, animals, the environment, everything. I'm assuming that libertarians believe that Adam Smith's invisible hand will take care of everything and make sure that all any company that doesn't do the community an overall service will not get business, but that's why I say they're idealistic.
Even with immense amount of regulations we have today, there are still many corporations who do evil, some of which even do more evil than good. You just can't microlegislate decency itself. Just imagine how much worse it would be with no regulations..
There are plenty of avenues for corporations do to evil and still get business, either because people are unaware of their wrongdoings or because they don't care. Even if people were conscientious enough to boycott corporations for wrongdoings, which they're generally not, it's illogical for the individual to do so because one person's refraining from buying goods/services from a company isn't enough to change that company's policies or to put them out of business, and yet it would definitely make things less convenient for the consumer who wants or needs that particular product/service from that particular company. And while mass boycotts are effective sometimes, the individual only has control over their own purchases, not a mass number of people's. The problem is similar to the paradox of voting, https://en.wikipedia.org/wiki/Paradox_of_voting . And again, people aren't conscientious or aware enough to boycott most of the times when it would be called for anyway.
Yes, libertarianism is in some sense a call for freedom, but it's freedom for the wrong agencies. We need rules for society to function as a civilization (such as a low that you cannot murder people), the question is only what rules we should have and how many. Libertarianism calls for too few rules and favors the wrong part of society.
I wrote more about the subject of libertarianism on Quora, covering a few points I didn't here: https://www.quora.com/Why-do-you-personally-disagree-with-classical-liberalism-libertarianism