Before Christ, there were the Jews already, who did worship the true God aside from their times of rebellion and apostasy.
Yes. All non-Christian religions that have deities are worshipping demons.
Yes bro. Demons see worshipping like that as an opportunity to be worshipped and fool people. "Do you want to worship that Odin God you just came up with? No problem, call me Odin, call me Ra, call me Vishnu, call me Moloch, call me Iemanjá, whatever floats your boat. Just worship me instead of the true God".
A lot of violent stuff was done in the name of Christ, especially by the Roman "catholics". But this says nothing about God and everything about men. In the bible we are not told to wage war and conquer pagans by sword, but rather to preach the gospel to non-believers. Most "Christian" man-made denominations, including the biggest of them, the Roman one, are cults.
Matthew 7 KJV
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.