It was a Disney villain death. Nothing more. It was a dark moment, but was there to highlight the difference between the naive Superman and the more cynical Heroes. Superman would have dragged him to the Hague, but does Hawkgirl even know about that? I would like to see something like that going forward in the series, but it makes sense now.

https://tvtropes.org/pmwiki/pmwiki.php/Main/DisneyVillainDeath