First and foremost we Catholics believe we were created by an Eternal Loving God who has done everything for us; He has grown our food for us; He placed out planet the correct distance from the sun; he holds our atoms together ect. We can do nothing in return to merit this, all He asks of us in return is to have a loving relationship with Him and with His children, our fellow human-beings. To worship figments of our imagination, God-hating demons or personifications of temporal things is deeply heart-breaking and treacherous to the God who created us.
Now some of the gods of paganism might be demons appearing to shamans, oracles and false prophets to lead them astray and some might have been invented by people to explain certain unanswerable questions in their society. However most of the pagan gods are in fact personifications of temporal goods; sex, money, social stability ect. when these things are idolized they lead to an addictive lifestyle, let me refer you to some sermons by the Protestant pastor Timothy Keller who best explains these sins and flaws of Idolatry:
youtube.com/watch?v=OOHdk3FR5Hg
youtube.com/watch?v=_mK65lpveSM
You might also want to check out his book Counterfeit gods which elaborates on these sermons
Finally most forms of Paganism not only show contempt and ingratitude for the Creator but contempt and ingratitude for Creation (including Zorastrianism, Wiccanism and most forms of Buddhism) they view some or all of Creation as evil or chaotic and not as a beautiful gift of God that should be embraced and respected.