This is because a murderer kills the body, but someone who commits shirk, associating partners with Allah in worship, is supposedly killing the soul.
“Australian Salafi Scholar Abu Ousayd: Saying ‘Merry Christmas’ Is Worse Than Congratulating A Person For Murder, Adultery, Drinking, Gambling,” MEMRI, December 16, 2022:
Australian Salafi scholar Wissam “Abu Ousayd” Haddad said in a December 16, 2022 lecture that saying “merry Christmas” is “more severe and hated by Allah” than congratulating someone for drinking alcohol, committing murder, committing adultery, or gambling. He added that this is an act of disbelief that sparks Allah’s anger. The lecture was posted to the YouTube account of the Al Madina Dawah Centre, which is located in Sydney, Australia. For more about Abu Ousayd, see MEMRI TV Clips Nos. 9643 and 9483.
Wissam Abu Ousayd Haddad: “Congratulating them [on non-Muslim festivals] is more severe and hated by Allah than congratulating someone for drinking alcohol, for committing murder, or committing adultery, and so on. Many of those who have no respect for this religion fall into this error. They do not realize the ugliness of their actions. A person who congratulates for sin, is [committing] a bid’a [forbidden innovation] or an act of kufr [disbelief], and would be liable to the anger of Allah.[…]
“It is worse to say to somebody ‘merry Christmas,’ than to say to somebody who was committing fornication, drinking alcohol, gambling, doing all those things that are major sins… It would be better – not that you should do it…
“It would be better to say to somebody ‘happy drinking,’ than to say to somebody ‘merry Christmas.’”