On the Creation of Tulpas, by Alexandra David-Neel Options

On the Creation of Tulpas
However interested we may feel in the other strange accomplishments with which Tibetan adepts of the secret lore are credited, the creation of thought forms seems by far the most puzzling.
...
Phantoms, as Tibetans describe them, and those that I have myself seen do not resemble the apparitions, which are said to occur during spiritualist séances.

As I have said, some apparitions are created on purpose either by a lengthy process resembling that described in the former chapter on the visualization of Ydam or, in the case of proficient adepts, instantaneously or almost instantaneously. In other cases, apparently the author of the phenomenon generates it unconsciously, and is not even in the least aware of the apparition being seen by others.

However, the practice is considered as fraught with danger for every one who has not reached a high mental and spiritual degree of enlightenment and is not fully aware of the nature of the psychic forces at work in the process.

Once the tulpa is endowed with enough vitality to be capable of playing the part of a real being, it tends to free itself from its maker¹s control. This, say Tibetan occultists, happens nearly mechanically, just as the child, when his body is completed and able to live apart, leaves its mother¹s womb. Sometimes the phantom becomes a rebellious son and one hears of uncanny struggles that have taken place between magicians and their creatures, the former being severely hurt or even killed by the latter.

Tibetan magicians also relate cases in which the tulpa is sent to fulfill a mission, but does not come back and pursues its peregrinations as a half-conscious, dangerously mischievous puppet. The same thing, it is said, may happen when the maker of the tulpa dies before having dissolved it. Yet as a rule the phantom either disappears suddenly at the death of the magician or gradually vanishes like a body that perishes for want of food. On the other hand, some tulpas are expressly intended to survive their creator and are specially formed for that purpose.
...
Must we credit these strange accounts of rebellious "materializations", phantoms which have become real beings, or must we reject them all as mere fantastic tales and wild products of imagination?

Perhaps the latter course is the wisest. I affirm nothing. I only relate what I have heard from people whom, in other circumstances, I had found trustworthy, but they may have deluded themselves in all sincerity.

Nevertheless, allowing for a great deal of exaggeration and sensational addition, I could hardly deny the possibility of visualizing and animating a tulpa. Besides having had few opportunities of seeing thought-forms, my habitual incredulity led me to make experiments for myself, and my efforts were attended with some success. In order to avoid being influenced by the forms of the lamaist deities, which I saw daily around me in paintings and images, I chose for my experiment a most insignificant character: a Monk, short and fat, of an innocent and jolly type.

I shut myself in tsams and proceeded to perform the prescribed concentration of thought and other rites. After a few months the phantom Monk was formed. His form grew gradually fixed and lifelike looking. He became a kind of guest, living in my apartment. I then broke my seclusion and started for a tour, with my servants and tents.

The Monk included himself in the party. Though I lived in the open, riding on horseback for miles each day, the illusion persisted. I saw the fat tulpa; now and then it was not necessary for me to think of him to make him appear. The phantom performed various actions of the kind that are natural to travelers and that I had not commanded. For instance, he walked, stopped, looked around him. The illusion was mostly visual, but sometimes I felt as if a robe was lightly rubbing against me, and once a hand seemed to touch my shoulder.

The features which I had imagined, when building my phantom, gradually underwent a change. The fat, chubby-cheeked fellow grew leaner, his face assumed a vaguely mocking, sly, malignant look. He became more troublesome and bold. In brief, he escaped my control. Once, a herdsman who brought me a present of butter saw the tulpa in my tent and took it for a living lama.

I ought to have let the phenomenon follow its course, but the presence of that unwanted companion began to prove trying to my nerves; it turned into a "day-nightmare". Moreover, I was beginning to plan my journey to Lhasa and needed a quiet brain devoid of other preoccupations, so I decided to dissolve the phantom. I succeeded, but only after six months of hard struggle. My mind-creature was tenacious of life.

There is nothing strange in the fact that I may have created my own hallucination. The interesting point is that in these cases of materialization, others see the thought-forms that have been created.

Alexandra David-Neel
Magic and Mystery in Tibet.
University Books Inc., 1965

Source

I've read this story a couple of times also, and I think I know why her tulpa became "mischievous". She never gave it a reason for existing, she just created it and let it walk around. And then on top of that she also had the preconceived notion that all tulpas turn against their creators, so it was almost inevitable that it would happen over time.