Better late than never....
Now I'm wondering just what exactly it takes to kill guys like them. I mean, does getting impaled through the heart really work on something from which "amorphous blob" is not entirely an unfair assessment of their natural state? Maybe it was sheer exhaustion and trauma that really did the deed--which is actually kind of terrifying.
Ah, an anatomy question, my FAVORITE!
So, there's a couple things going on. Trauma is, yes, a big part of it. A kitik isn't as much of a 'blob' as it looks. It's more of a ball of fluids in a muscle container. A thick layer of integumental tissue filled with stacked color cells covers the outside: think of these cells like a game of jenga. When sent a neural signal they slide out the right layer of cells to reflect a specific wavelength of light, changing the kitik's color. In their relaxed state these cells revert to tidy stacks which reflect all colors, creating the silvery appearance. That integument covers several thin layers of heavily innervated muscle tissue in which every single muscle cell has its own nerve, which allows for the amazing control required in mimicry. Under that is a thick layer of neural tissue, and at the very core of the creature is the ichor reservoir attached to millions of microscopic tubes that passively pump blood with every muscular movement, keeping the kitik constantly oxygenated and all systems working.
This pertains to the question because it's actually pretty easy to kill kitik: just punch a hole through their core and keep it open long enough for them to lose most of their ichor (blood), or force them into a shape so small and dense that their surface to mass ratio is too high for effective oxygenation and they suffocate. The more a kitik has exherted itself, the less it has been able to oxygenate itself through its skin (and the less attention it has had for complex structures like lungs) so a tired, traumatized kitik is already in danger of oxygen deprivation, and the loss of even some oxygenated blood will kill them in that state. Cold is also a killer: the colder they are, the less supple and able to oxygenate the blood their integumental layer is. Below about negative 15 F they can't survive at all.
Now a pistol bullet won't do the job of exsanguination, because a kitik can patch the wound with other layers of muscle and heal it later, but skewer one or use a large caliber bullet? They're dead.
Wow, what an in-depth answer! You really did your research with this stuff. I'm impressed! And it's interesting to hear that kitik aren't as invincible as I thought. I just assumed something that can change shape so easily would probably be hard to kill.
Most people do. Of course, a lot of people think they eat babies too... the misunderstanding creates a lot of very paranoid kitik ;)