Synx wouldn't be quite so dangerous if they were little more than the grotesque, animated stomachs most view them as. Unfortunately these eels are capable of demonstrating surprisingly advanced reasoning and cognitive skills, to a level that could perhaps be put on par with a human being. They might not know mathematics or philosophy, but one can get into a secure house with the equivalent ease of a serial killer doing the same task and solving the same problems. But while seemingly a human's near equivalent in regard to raw brainpower, there really isn't anything human about them. Intellectually they forever reside in a rather uncomfortable chasm: an uncanny valley of thought encompassing everything detached from what humans understand as thinking. They are more or less unknowable, vile in every animalistic way and outright incapable (or unwilling) to understand concepts such as mercy, apathy or morality of any kind.
Nowhere is raw synx intelligence more measurable than in their observation skills. Appearing beast-like, dimwitted and nearly inanimate to most casual observers, keen eyes will immediately notice the exceptional concentration synx put towards watching the things they intend to eat. A particularly sharp synx will take extraneous note of a potential victim's reaction to sound, sense of smell, blind spots in vision as well as their walking speed, weight, outward appearing strength, time of sleep and when they're alone. When synx attack dangerous meals, they usually only do so in an instance they believe they will win. Synx are not looking for a fight, they are looking to get fed. While children offer little to no resistance, a hungry synx will still have to use that acute observation ability on the guardians of such a meal. These inanimate-looking animals probably use as much brainpower in a day as the average person, they have to in order to successfully eat the average person.
While not all synx can, it is common to hear one try and speak. It quickly becomes clear though that this is not communication, but instead the mere mimicry of sound. Given the words they choose to use, it is obvious they have no idea what speech actually is, even if they are aware it's communication of some kind. The most common things uttered are the cries and pleas of those they've attacked and consumed. It is thought that synx simply lack the type of wiring needed to regulate and understand verbal communication. In areas where synx are abundant, well known and seen as mere nuisances, rumors and tall tales circulate about synx who can genuinely communicate. Of course it always turns out to be a hoax of some kind. No synx displaying this talent has ever been officially logged, captured or held in captivity by a single alien species that has encountered them, and such stories are dismissed as imaginative or obvious fabrications created by attention seeking individuals.
While you may not be able to talk with these baby eating space-fish, communication through incentive has been proven to work quite well. In captivity, synx have been seen responding almost immediately to both positive and negative reinforcement. When a synx's captors want it to enact a particular behavior, synx usually oblige when pain is made a clear consequence of poor performance. Performance also increases dramatically when food is offered as a reward. Outside of testing environments, records of unsavory individuals keeping these animals as pets tell a similar story. Entirely for its own benefit, a synx will form an unspoken truce with someone who hurts or kills other people. Assuming that person won't harm the synx, and assuming the synx does not believe they can eat this person, the union between the two can be seen as an alliance, especially if this "bad person" is feeding the synx the people they harm. Such incidences show that these creatures are indeed capable of being bargained with, even if it is on the most basic, animal-level of self interest.