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
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?"
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.