What do the suttas say about such people? Surely they're acting on their ignorance (dependent origination), but can't they make use of restraint/self control?
Is there no penalization in such cases according to the Dhamma? If murderers show no regret, then they aren't even born in hell right now.
The suttas say murderers are taken before the King and get executed. The Buddha had equanimity towards this; as shown in many suttas.
Very well then, Aggivessana, I will cross-question you on this matter. Answer as you see fit. What do you think? Would a consecrated, noble-warrior king — such as King Pasenadi of Kosala or King Ajātasattu Vedehiputta of Magadha — wield the power in his own domain to execute those who deserve execution, to fine those who deserve to be fined, and to banish those who deserve to be banished?
Then the Blessed One said to him, "Answer now, Aggivessana. This is not the time to be silent. When anyone doesn't answer when asked a legitimate question by the Tathāgata up to three times, his head splits into seven pieces right here."
Now on that occasion the spirit (yakkha) Vajirapāṇin [Thunderbolt-in-Hand], carrying an iron thunderbolt, was poised in the air above Saccaka the Nigaṇṭha-son, (thinking,) "If Saccaka the Nigaṇṭha-son doesn't answer when asked a legitimate question by the Blessed One up to three times, I will split his head into seven pieces right here."
MN 35
For example, in the Angulimala Sutta, when the King threatened to capture & execute the serial killer Angulimala, the Lord Buddha did not exhort the King to not do so. Instead, the Buddha said to the King: "What would you do if I rendered Angulimala harmless?"
Then King Pasenadi Kosala, with a cavalry of roughly 500 horsemen, drove out of Savatthi and entered the monastery. Driving as far as the ground was passable for chariots, he got down from his chariot and went on foot to the Blessed One. On arrival, having bowed down, he sat to one side. As he was sitting there, the Blessed One said to him, "What is it, great king? Has King Seniya Bimbisara of Magadha provoked you, or have the Licchavis of Vesali or some other hostile king?"
"No, lord. King Seniya Bimbisara of Magadha hasn't provoked me, nor have the Licchavis of Vesali, nor has some other hostile king. There is a bandit in my realm, lord, named Angulimala: brutal, bloody-handed, devoted to killing & slaying, showing no mercy to living beings. He has turned villages into non-villages, towns into non-towns, settled countryside into unsettled countryside. Having repeatedly killed human beings, he wears a garland made of fingers. I am going to stamp him out."
"Great king, suppose you were to see Angulimala with his hair & beard shaved off, wearing the ochre robe, having gone forth from the home life into homelessness, refraining from killing living beings, refraining from taking what is not given, refraining from telling lies, living the holy life on one meal a day, virtuous & of fine character: what would you do to him?"
"We would bow down to him, lord, or rise up to greet him, or offer him a seat, or offer him robes, almsfood, lodgings, or medicinal requisites for curing illness; or we would arrange a lawful guard, protection, & defense. But how could there be such virtue & restraint in an unvirtuous, evil character?"
MN 86
In other words, the Buddha was not a secular law-maker, such as Moses or Mohammed (PBUH) were. The impression is it is the role of the secular society (and not the Buddhist Sangha) to determine its criminal law. If secular law-makers decide to follow the example of the Buddha to attempt to reform criminals, its up to those lawmakers.
By letting law enforcement do their thing?
Regret is not needed for the consequences of your actions to happen.
I think the suttas have two or three messages, addressed to each of us as individuals:
Sometimes a counter-argument to doing something is, "but what if everybody did that?" For example maybe it's not very harmful if one person cuts down a tree, but what if everybody does?
But these rules, I think they generalise -- i.e. if everyone was like that it would still be no problem, no harm.
I think that the Buddhism of the suttas though isn't primarily laws about how to govern lay society.
There's one more thing, I read that the Buddha never approved killing -- ever -- except killing anger (and therefore wouldn't approve of executing serial killers).
When asked if there was anything whose killing he approved of, the Buddha answered that there was only one thing: anger. In no recorded instance did he approve of killing any living being at all. When one of his monks went to an executioner and told the man to kill his victims compassionately, with one blow, rather than torturing them, the Buddha expelled the monk from the Sangha, on the grounds that even the recommendation to kill compassionately is still a recommendation to kill — something he would never condone. If a monk was physically attacked, the Buddha allowed him to strike back in self-defense, but never with the intention to kill. As he told the monks,
"Even if bandits were to carve you up etc.
I think that Buddhist countries (like Tibet) tended to impose non-lethal punishments on criminals.
Its seems like your ideas about Tibet are the Hollywood version That was from the book, Seven Years in Tibet. The Wikipedia article you linked to says "Because Tibetan Buddhism prohibits killing..."