My guess is that 666 (with 6 being the number of “man”) stands for the worship of man rather than God. The beast will institute a world government that finally puts into practice the humanistic ideas that we don’t need God, man can be gods themselves ........... 666 = man-man-man (humans who think they have evolved into petty little gods)
And the scripture in Revelation pretty much sums up God’s reaction to that!
If I recall my Biblical teaching, 666 was demonic response to Father, Son and Holy Spirit. It is where the Satan, the anti-Christ and the false prophet are in charge.