If I leave out the wp_die() in the testiframe function I get the ‘0’ appended to my output. If I put it in, the page w/ iframe linked to it crashes

add_action('wp_ajax_ajtestiframe','testiframe');

<?php
function testiframe() {
?>
    <head>
    <style>
    .aatest1 {color:red;}
    </style>
    </head>
    <span class="aatest1">This is a test</span><br>
<?php
    echo 'This is my iframe content<br>';
    var_dump($_GET);
}
    wp_die();
?>