The subject of hell disturbs everyone, except those who don't believe in it! Given that nobody warned more strongly about the need to avoid hell than Jesus Christ, it's important to study his words on the subject. After all, he should know better than any human what hell is all about! Read Mat. 5:21-30; 10:26-28; 13:40-43; 18:7-9; 23:33; 15:4; Mark 9:43-44; Luke 12:4-6; 16:19-31; Rev 20:11-15. In Revelation, where Jesus is spoken of as 'the Lamb', we are told about all those judged adversely by God on the Day of Resurrection and Judgment:
"And he shall be tormented with fire and brimstone in the presence of
the holy angels, and in the presence of the Lamb; 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" (Revelation 14:10-11).
This happens after an angel has proclaimed the everlasting gospel to every nation, kindred, tongue and person on earth. They are commanded to fear God, to give glory to him, and to worship him, for the hour of God's judgment has come (14:6-7). So, before this hellish awfulness is poured out on those ones, they have the chance to avoid it by responding to the preaching of the everlasting gospel.
Now, what about God's mercy? The Bible states in James 2:13:
"For he shall have judgment without mercy that hath shewed no mercy :
and mercy rejoiceth against judgment."
In other words, if you want to have God show mercy to you, show mercy yourself to others. Mercy exults over judgment, but God's judgment is always righteous. This means that when push comes to shove, those who trample on God's patience and mercy will experience his righteous judgment, and end up in hell. With nobody to blame but themselves.
Now, this is where your comments show the tendency of those who do not really understand the righteousness of God. You said, "I would not condemn anyone to hell, even villains like Hitler and Stalin. Perhaps I’d have them spend a long time being purged and making up their faults, but at the end I’d let them into eternal happiness, assuming they want to be there". If you think that, after physical death, there is a place for souls to be purged - purgatory - then you need to ask yourself, "Why then, did Jesus have to suffer and die? If what he did was insufficient to spare people hell, given there's supposedly another place where they could eventually get out of purgatory and into heaven, what does that say about Jesus being insufficient to save sinners?"
During his Hell's Best Kept Secret teaching, Ray Comfort speaks of people who say things like
"I don't believe that a loving God would send me to hell. My God is a
God of love, not wrath and judgment." Ray says "If they ever tell you
that, say, 'You're right. Your god would never send you to Hell
because he can't. He doesn't exist. He's a figment of your
imagination. You created a god in your mind in your own image. That's
called idolatry, it's the oldest sin in the book, and all idolators
are condemned to Hell.'".
Likewise, regarding Rev. 14:10, John Metcalfe writes in The Revelation of Jesus Christ p392 (The Publishing Trust 1998) – now only available through Amazon :
“He shall be tormented with fire and brimstone… smoke of their torment
ascendeth up for ever and ever…” ‘Tormented’ At its strongest, this
word means ‘tortured’. With what torture? ‘Fire and brimstone.’ ‘Upon
the wicked he shall rain snares, fire and brimstone, and an horrible
tempest: this shall be the portion of their cup’, Psalm 11:6. Thus it
fell upon Sodom and Gomorrah, a sign of the wrath to come. But in the
wrath to come it falls out of eternity, unto everlasting, ‘the
vengeance of eternal fire’, Jude 7.
"Oh, says one, This is too terrible; too severe. I answer, Then heed
the first angel, and obey him, if you would avoid its severity. Oh,
no, comes the reply, But I could never worship such a God as this. But
I answer and say, Then who or what do you worship? Oh, you cry, But my
Jesus, and my Spirit, are nothing like this! Then what Jesus is this?
What spirit is this? It is ‘another Jesus’ and ‘another spirit’, just
as you hold ‘another gospel’ than that preached by the apostles at the
beginning, II Corinthians 11:4. The Jesus of John and the apostles
gives you the lie, saying, ‘I Jesus have sent mine angel unto the
churches’, and again, John’s Jesus ‘sent his angel to signify these
things’, namely the things in Revelation 14:10,11. As with all the
Book of the Revelation in every jot and tittle, called ‘The Revelation
of Jesus Christ’.
"Here the truth brings out the enmity of false religion. False
religion, under the guise of the acceptance of premillennialism and
futurism, beclouds these things, and takes away the word of the
Spirit... with their easy-believism... these Brethren prefer to charm
people, to avoid the negative, to obviate fear, hence they put the
ointment of soft sentiment and the convenience of disappearing ink, to
hide such places as Revelation 14:9-11. Look and see. Therefore in all
fidelity I am bound to call their system, Heresy."
The greatest mercy in the entire universe is to be found, alone, in God, its creator. His mercy is staggering, but his righteousness is never compromised. When the Day of Resurrection and Judgment comes, and everything hidden is revealed (Matthew 10:26-28) we will all see that the punishment of hell has been given to every single one that rejected the gospel of Christ and had contempt for God's righteousness. And they will have nobody else to blame but themselves.
There is no contradiction between God's amazing mercy and the Son of God's clear warnings about those who despise it, or those who contemptuously presume upon it.