Q:

How would anyone ever "worship idols out of love or fear"?

It says in the Gemara (Krisus 3a) that if one worships an idol without accepting it as a god, he isn't liable to even bring a chatas. This opinion seems to held according to everyone.
Yet, the Gemara says a few lines later that Abaye says that one is liable if one worships idols out of fear [i.e someone forced him to worship idols].
Presumably, even Abaye would agree that when one forces a person wot worship idols, he doesn't really believe in that idol. So why is he liable? 
Moreover, there is a Halacha that one must give up his life rather than serve idols. How could such a case ever happen, as he could be serving the idol without "taking it upon himself as a god"?

A:

It's not so straightforward that "out of fear" means that he was forced. Rambam (Hil. Avodah Zarah 3:6) explains "out of love" to mean that he is attracted by the beauty of the statue, and "out of fear" as that he thinks that it has the power to harm him (which doesn't necessarily mean that he accepts it as a god, just as a power of some kind).
Though it is true that most other Rishonim do understand "out of love" and "out of fear" as referring to one's emotions towards another person. Tosafos (Sanhedrin 61b, ד"ה רבא) draw a distinction between a deity that is commonly worshipped - where if a Jew then goes and does so too, then (according to everyone) the excuse that it was out of love or fear won't wash - and one which has just been established by royal decree or the like (the example in the Gemara there is Haman, where Achashverosh had ordered that everyone bow to him), where Abaye still holds that he's liable because he did actually worship, whereas Rava exempts him because he didn't accept it as a god.
[Regarding your last question, Tosafos there uses the above as one possible way to reconcile these two halachos. They also offer another resolution: one is indeed obligated to give up his life rather than serve an idol (any kind, whether a recognized deity or not), but if he didn't do so and worshipped it under duress, then according to Rava he would indeed be exempt from (court-imposed) punishment.]

