Yehoshua Perek 20
ו וְיָשַׁב בָּעִיר הַהִיא, עַד-עָמְדוֹ לִפְנֵי הָעֵדָה לַמִּשְׁפָּט, עַד-מוֹת הַכֹּהֵן הַגָּדוֹל, אֲשֶׁר יִהְיֶה בַּיָּמִים הָהֵם; אָז יָשׁוּב הָרוֹצֵחַ, וּבָא אֶל-עִירוֹ וְאֶל-בֵּיתוֹ--אֶל-הָעִיר, אֲשֶׁר-נָס מִשָּׁם.
He shall dwell in that city until he stand before the tribunal for judgment, until the death of the Kohen Gadol that shall be in those days. Then shall the slayer return, and come to his own city, and to his own house, to the city from which he fled. ז וַיַּקְדִּשׁוּ אֶת-קֶדֶשׁ בַּגָּלִיל, בְּהַר נַפְתָּלִי, וְאֶת-שְׁכֶם, בְּהַר אֶפְרָיִם; וְאֶת-קִרְיַת אַרְבַּע הִיא חֶבְרוֹן, בְּהַר יְהוּדָה.
They set apart Kedesh in Galilee, Mount Naphtali, and Shechem in Mount Ephraim, and Kiryas Arbah, which is Chevron, in the mountain of Yehuda.
The pesukim describe the locations of the arei miklat - Cities of Refuge for unintentional murderers. The Gemara (Makkos; 9b) asks: Why are there three arei miklat on the other side of the Yarden for 2½ tribes, yet only three for the rest of Israel- 9½ tribes? This ratio seems illogical? The Gemara answers that 'in Gilead (on the east bank of the Jordan) there are more murderers', therefore, there needed to be more arei miklat. However, this does not answer the original question – the arei miklat are for unintentional murderers, intentional killers are given the death penalty; if so, why do more intentional murderers require more arei miklat?
There are three approaches taken in answering this question: The Ramban answers that since there are more cases of murder, more people will claim that they killed accidentally. The Gemara in Makkos (9b) writes that all killers would initially go to the ir miklat and would then be processed according to their crime, whether intentional or unintentional. Thus, more arei miklat were required to accommodate the larger number of intentional killers who claimed to have killed unintentionally, before they were processed by Beis Din.
Tosfos (Makkos 9b) answer based on a different Gemara in Makkos (10b) which explains that Hashem will gather an intentional killer (who was not put to death due to lack of witnesses) and an unintentional killer (also, who murdered without witnesses) to one place. The accidental murderer will climb down a ladder and fall on the intentional killer sitting below, killing him. The intentional killer will have received his just deserved (death). There will be also be people present which will force the unintentional killer (now for a second time) to flee to an ir miklat. Thus, the people who are killed accidentally are actually those who murdered intentionally. If there are more intentional murderers, there will be more cases where they could not be punished, therefore, there will be more accidental deaths, requiring more arei miklat.
Finally, the Maharal (Gur Aryeh, Bamidbar 35:14) explains that one who committed a sin unintentionally is not completely blameless, and indeed needs to bring an offering to gain atonement for the sin. If a person was vigilant enough and cared enough about the sin in question, he would have been careful to avoid sinning, even unintentionally. A nuclear plant worker realises the importance of avoiding an accident, and is therefore extra careful when handling nuclear materials. Similarly, everyone who realises the severity and the nefariously base nature of murder, rape, or pillage would make absolutely certain never to commit such acts even unintentionally. One who realises the severity of such acts will make sure to avoid anything that might lead to those acts being committed, even unintentionally. Ultimately, even an unintentional sin requires atonement, for deep down it comes from a certain degree of disregard for the seriousness of the sin in question. An intentional murderer has desensitised himself to the act of murder. After he has committed this act, and especially if he repeats it, he does not regard murder with such gravity; it has become lighter and less abhorrent in his eyes. Thus, it is precisely due to his newfound disregard for the act of murder that he is less likely to avoid situations that might lead him to cause a death unintentionally. Whilst most of us would not climb a ladder under which someone is situated, this seasoned murderer would not think twice about climbing the ladder because he does not see as great a need to avoid such a situation as we do. Our realisation of the gravity of murder moves us to keep away from such situations. Thus, an intentional killer is more likely to kill unintentionally too.