Those that flee after the abomination that causes desolation are refusing the Mark of the Beast - they have to flee to avoid being killed.
1/3 are brought through the fire (Zechariah 13 if memory serves).
NOW, regarding the rest of the world, yes, anyone who accepts the Mark of the Beast is doomed - there is no repentance for that. They are tortured with fire and brimstone in the presence of the Lamb and the holy angels forever.
And ever.
Yikes.
Revelation could not be clearer on this warning. The 3rd angel is the one who says it:
(Scripture to follow)
Revelation 14:9-12
King James Version
9 And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
12 Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.
https://www.biblegateway.com/passage/?search=Revelation%2014%3A9-12&version=KJV