Scientology can (and has by other groups) be perverted and then used for evil purposes. By using altered Scientology materials, one can get themselves in quite a mental and spiritual mess. By having untrained persons use the technology on others, they can foul them up.
I have heard that the military asked about obtaining Dianetics for use in the cold war before it was released to the public. This use is known as "Black Dianetics". It was noted immediately that the same technology used to free man, could be abused to enslave him as a military robot, or worse. So L. Ron Hubbard created some precautions to prevent this from ever happening to members of the Church of Scientology.
I personally knew someone who discovered this altered incorrect "OT" technology on the internet and tried to apply it to himself and it basically drove him crazy, and soon after, he killed himself.
Noted squirrel (colloquial Scientology term for one who intentionally alters technology) "Captain" Bill Robertson developed a malignant throat cancer which led to his death.
Obviously, health can deteriorate and Scientologists do get sick and with age do die. But I don't know of any circumstances in which correct technology correctly applied has resulted in illness or death.
Additional answer from blog:
On "higher Scientology levels” and I give you my experience as an internet enthusiast and Scientologist. There is so called material on the internet which claims to be “confidential Scientology scriptures”. One of the guys who stole this material in the 1980s (they wanted to experiment with it outside of the church and make some money with it) said some years later that they “amended” and “corrected” it, i.e. it’s not the original material out there on the net. Scientologists are aware what can happen if you mess around with Scientology technology and do it the wrong way. Not that it kills you but it can get confused or very unhappy doing it the wrong way.
So what I am reading on the internet is at least out of context - which is the usual with material posted for the sole purpose to ridicule Scientologists - or at worst not even real Scientology material. It reads like science-fiction, or very old. Scientologists believe that they have lived before and that there are many earlier lives. The higher Scientology levels include bits and pieces about past lives going back to times before planet earth. I believe and that is common for Scientologists that this data is historical and might or might not influence the condition of people today. But to make up your own mind of this is true or not you will have to read the full story in sequence, with training, practice and all the background there is. Computer Science 3 is tough or impossible to understand for a beginning HTML coder (ok, Computer Science is not a religious belief, but you get the point?).
Finally a personal thing: some time ago I got a flyer with a printout of this “material”. The guy who distributed it gave it only to Scientologists coming from Sunday service in a Church of Scientology. The flyer was ridiculing all and any belief of Scientology. Now, what purpose did this have? Intolerance. I was about to smack the guy, only that my friend was holding me back. Here comes some dude with no clue about Scientology but some ridiculous internet printout and tries to make fun out of my beliefs and those of my friends? Man, this is how wars start, religious wars. This is pure intolerance and provocation, a violation of my basic rights. Do I go to a Catholic service and call them cannibals because they just drank “the blood of Christ”? Do I go to such a service and distribute flyers about some things in Christianity which you can only believe but not see? No, I don’t, because I believe that no one has a right to destroy the spirituality of others, especially if it makes them help and care for others, as Scientology does, and as many others religions do.