Proper translation of 妖怪?
For my information, 妖 is related to ...uhm temptress? Which means an alluring woman who seduces or exploits men OR a woman considered sexually attractive by men. Often being used on female. Normally 妖 is able to control someone's (male's) mind.

While 

怪 means something is really peculiar or weird. 

妖物 or 妖怪 was born from a temptress and a peculiar thing, it does have its own mind or negative thoughts and extraordinary abilities. Hence, we called them monster, goblin, devil and so on，while spirit is just a force or energy which does not have a physical or virtual body and cannot do anything (don't confuse with ghost).

Should it not be a combination of spirit and monster?
Answer: Well, it's so so. People may say yes, while people may say no.

Spirit is meant by 靈魂 / 幽靈 (meaning approached to "ghost").

Well, a combination of spirit and monster, I can't actually explain in full in English. But you can watch this movie: See Here

to know more about the spirit and monster hybrid.

A combination of spirit and monster, something like a spirit (more like ghost) conquers a monster's body which the monster's body does not have mind / thoughts.

Hope I did answer your question.