When my rosh yeshiva met the Rebbe many years ago he asked “a lot of your chasidim are saying that you are moshiach, are you moshiach?”
The Rebbe slammed on the table and said “chasve shalom”. My RY proceeded to tell me that the reason the Rebbe never visited eretz yisroel was to dispell this notion.
The chasidim who call him moshiach or God are doing it from an emotional standpoint. A lot of them have experienced open miracles and yeshuas from his advice and brachos that have changed their lives. They don’t have anyone else in their lives currently who can work wonders in our day and age, so they grasp to the only person who was able to help them and hope he can save us all. This obviously isn’t the correct ideology, but I hope it explains the fanaticism happening. Not all lubavitchers believe he’s moshiach, but for the ones that do, it’s not the rebbe’s fault, he didn’t promote this belief system.