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 Beis Din acting extrajudicially) or make his life a misery as we see in [The Rambam where he makes the following statements:][1]



> 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."


  [1]: https://www.sefaria.org/Mishneh_Torah%2C_Murderer_and_the_Preservation_of_Life.2.4-5?lang=he