If you read my post, I included: Unlike becoming a vampire, which can happen unexpectedly, you must CHOOSE to become a lich. And, vampires by nature retain some of their human emotions and thoughts, just as werewolves transform back into humans. Liches, however, allow their humanity to corrupt and decay completely.
That having been said, there was a lich in Bloodmoon who was not-so-evil, and a lich in Tribunal who I think you had friendly dialogue with. Either way, they had to at least have done something wrong, otherwise you wouldn't have been seen to find them.
Lichdom is done for one of two reasons, power and the longevity of being one. Now, we've have seen some benevolent liches (BM and Tribunal's liches). They chose the road to lichdom to pretty much protect people by increasing their lifespan. Then there's the other road to basically increase their power and the immortality with it. That group tends to attract amoral mages, those who want to extend their life and/or wield greater magics by living longer, or they want to live forever, be more powerful, and drown kittens because they are evil. It is not inherently evil, it just seems that way.
For one, I don't think it completely decays and corrupts ones humanity. As we have seen, the two benevolent liches held on to their humanity. The other kinds already gave it up. With Mannimarco, for example, he was already amoral, and his ascension to lichdom was only to extend his life forever so he can find a way to give Arkay the middle finger some day. Either way, blatantly saying all liches are evil is too much of a generalized statement.
With all that said, all liches we have seen remained in hiding, for no one would just let them wander around. Even Mannimarco would not just take a stroll down the street. You had to come to him, or beckon him too you if you wanted to strike a deal. The process is irreversible, mostly because those who do become liches in the first place do not want to go back to being mortal and die. Plus, no one has really found a way to reverse your body from a lifeless husk to not a lifeless husk