"Penalty" is the key word here. Like, issued by a judge, after proper judgement according to the law of the land. Not random shooting people without any due process.
This will definitely get lost in the conversation but like I said right up front, I dont actually agree with killing them. It seems that we ought to be able to intercept these boats and process them as suspects of a crime. It just rubs me the wrong way how every issue gets written up as a one-sided narrative of good vs evil depending on who you support politically.