I doubt very much if you killed this kitten. If you had lain on it long enough to kill it the kitten would be flat as a pancake. Kittens die... A huge proportion of feline young just do not make it.
I have seen four healthy looking kittens in the box with mum one minute... carried on ironing and within 5 minutes seen a dead kitten lying exactly as you described for no apparent reason at all. Young kittens have many problems that cannot be seen or improved... they may get too hot or too cold and if they are not actually feeding well the latter will ensure they cannot thrive, They can have a cleft palate which prevents feeding or an internal malformation. Sometimes the ribcage is malformed and this often gives the appearance of being crushed when the kitten is dead. Don't beat yourself up... it really was not your fault.
