Nah they're fine, and i guess I can see where you are coming from. It's true golems are controlled by magic, but its not always a way to decide what type of magic. This one would most likely have been controlled by something else, but even then it can be magic that was given to the golem by something to protect something else, even after whoever gave it the magic died. Then there are golems that are controlled by "wild" magic in a sense, which is really just an embodiment of the living energies of the organisms and nature around it. Ranzatsu is acting on what he believes, by his personality and his secondary tamer skill, which lets him think that even if they are digital creatures, the monsters do have a will and most likely have a reason for what they do.
Rare monsters seem to spawn randomly or specifically in various locations. Though, the golem was controlled by magic (generally golem's are), but I would think that if it wasn't a random spawn spawn then it wasn't simply protecting an object on his own and probably had a controller, probably a boss, sent the golem once they noticed an explorer at the entrance (or created the golem for that purpose, even if the creator doesn't exist anymore). I really couldn't see a monster that strong and rare being sent to the entrance unless the boss at the end of the cave was a [much] higher level, since it'd be rather odd for the strongest monster to be sent to face those who just entered. The golem was only about level 15 (give or take), and it's not so unbelievable that Akihiko would make a cave, and a somewhat noticeable one at that, a death trap.
So I guess I'm with you, then. I was actually reluctant to say anything because I had considered it, and from a logical point of view it being a guardian made sense, but...I guess I was partially undecided on the idea and needed a little push. And sorry about these long messages, this'll hopefully be the last one -.-
Just a heads up, since the monster wasn't a boss, and since our characters were in a party, the rewards should (or could) be about equal between each person. It's really not mandatory, it's just a suggestion.
Also, though this doesn't have to matter as far as in character knowledge, we were fighting him somewhat near the entrance to the cave. Not so close that we were almost literally outside, but close enough that one could definitely see the light and run away if need be. The only reason I'm saying this is because I'm not sure if the golem would come from the depths of the cave to the entrance if it were protecting something, that'd make whatever it is protecting rather rather easy for others to steal. That's not to say there isn't anything rare in the cave, there probably is, but given that the Golem was a rare monster, I'm not so sure that it was the guardian of the cave.
You can really make Ranzatsu do what he wants, I'll just go with what you decide. Though, I don't think trying to tame the golem is a good idea for two main reasons. The first reason being that, due to its being both rare and powerful, it probably won't work at your level (or mine), and the second reason being that, at the levels we are, if we actually were able to tame it then we would become overpowered. Plus I'd assume it'd drop some pretty good items, so if we can then I suggest killing it.