We have the earlier example of the execution of Noah in Ether 7:18 to show that the capture of the king was not always the option. In that case, Noah was killed as part of the rescue of Shule, a rescue that saved Shule, but also left Noah’s kingdom and powerbase intact.
In the current case, there is the opportunity to slay Jared, but Esrom and Coriantumr elect not to. It would appear that they are able to mount a successful coup and control the government. It is therefore probable that maintaining him alive had some political value. As it turns out, it also had its risks.