Someone who does not meet the requirements of the Beis Din on order to get the death penalty has one of two options; either killed by the king or make his life a misery as we in The Rambam where he makes the following statements:
https://www.sefaria.org/Mishneh_Torah%2C_Murderer_and_the_Preservation_of_Life.2.4-5?lang=he
וְכָל אֵלּוּ הָרַצְחָנִים וְכַיּוֹצֵא בָּהֶן שֶׁאֵינָן מְחֻיָּבִין מִיתַת בֵּית דִּין. אִם רָצָה מֶלֶךְ יִשְׂרָאֵל לְהָרְגָם בְּדִין הַמַּלְכוּת וְתַקָּנַת הָעוֹלָם הָרְשׁוּת בְּיָדוֹ. וְכֵן אִם רָאוּ בֵּית דִּין לְהָרְגָם בְּהוֹרָאַת שָׁעָה אִם הָיְתָה הַשָּׁעָה צְרִיכָה לְכָךְ יֵשׁ לָהֶם רְשׁוּת כְּפִי מַה שֶּׁיִּרְאוּ: הֲרֵי שֶׁלֹּא הֲרָגָם הַמֶּלֶךְ וְלֹא הָיְתָה הַשָּׁעָה צְרִיכָה לְכָךְ לְחַזֵּק הַדָּבָר הֲרֵי בֵּית דִּין חַיָּבִין מִכָּל מָקוֹם לְהַכּוֹתָם מַכָּה רַבָּה הַקְּרוֹבָה לְמִיתָה לְאָסְרָם בְּמָצוֹר וּבְמָצוֹק שָׁנִים רַבּוֹת וּלְצַעֲרָן בְּכָל מִינֵי צַעַר כְּדֵי לְהַפְחִיד וּלְאַיֵּם עַל שְׁאָר הָרְשָׁעִים שֶׁלֹּא יִהְיֶה הַדָּבָר לָהֶם לְפוּקָה וּלְמִכְשׁוֹל וְיֹאמַר הֲרֵינִי מְסַבֵּב לַהֲרֹג אוֹיְבַי כְּדֶרֶךְ שֶׁעָשָׂה פְּלוֹנִי וְאֶפָּטֵר:
When a Jewish king desires to slay any of these murderers and the like who are not liable for execution by the court - by virtue of his regal authority, in order to perfect society, he has the license. Similarly, if the court desires to execute them as a result of a immediate fiat, because this was required at the time, they have the license to do as they see fit. If the king did not execute them, nor did the court deem the time as such to require strengthening the strictures against murder, it should nevertheless have the murderer beaten with severe blows - so that he is on the verge of death - and imprisoned, deprived and afflicted with all types of discomfort in order to strike fear and awe into the hearts of other wicked men, so that this death should not be a stumbling block and a snag for them, causing them to say: "Let me arrange to have my enemies killed the way so-and-so did, and I will not suffer the consequences."