Spaun sees a series of digits: 1 2 3; 5 6 7; 3 4 ?. Its neurons fire, and it calculates the next logical number in the sequence. It scrawls out a 5, in legible if messy writing.
What a performance! Now it only needs to control the 32.7 trillion cells in the body, mantain a thought process which features ideas, feelings, dreams, past experiences, personality, insticts, prevent the body from collapsing due to gravity etc. etc. etc. all at the same time.
A pure logical system will never do anything more than to logically follow instructions. It will do exactly what we instruct it to do, no matter how advanced and if it does wipe us out, it won't be "its" decision, it will be the programmers'.