It's fucking rude to do that at a Christmas market, antisocial, negative and provocative. People don't want loudly chanting flag waving men at a Christmas market, same as football hooligans marching around would be unwelcome. It's completely tone deaf and disrespectful of people who want to enjoy themselves.
Christmas markets are places of fun, peaceful, sociable experiences in the lead up to Christmas. Maybe not in the Uk where it's mostly commercial, but certainly in the rest of Europe.
While most visitors to Christmas markets today may not be active Christians, the tradition is closely tied to Advent and the coming of Christ. Looking further back, midwinter was celebrated as part of the Yule festival in many parts of Europe. The Anglo-Saxons marked the solstice with feasts to encourage the return of the sun, often including roasted meats, ale, and other traditional foods.
Pre-Christian winter festivals were communal events celebrating the return of light, with feasts, music, and entertainment. This sense of community celebration lives on in Christmas markets, where visitors enjoy seasonal treats, hot drinks (mulled wine), and festive foods, many of which have their origins in traditional midwinter feasts. Christmas markets have strong roots in these ancient solstice celebrations, they simply evolved from them.
So attacking a Christmas market is essentially attacking European culture as it has evolved over thousands of years. It's nothing to do with religion and all to do with culture and location. That psychiatrist killer is either a sleeper and has faked his anti muslim activities (doubtful) or he is a crazed terrorist who thought it's ok to attack the country that has given him a sanctuary, a calculated hateful, arrogant murderer, and evil crazy evil man who murdere people because he didn't get his own way. He's definitely a terrorist. Hope he rots in hell.