Hiccstrid is crazy because it's Hiccup "would kill for her" Haddock and Astrid "would die for him" Hofferson, not the other way round.
Yeah in any other case it's Hiccup 'would die for a cause/person' and Astrid 'would kill for cause/person' but when it comes to each other? Hiccup has canonically been ready to kill for Astrid, as in blast-a-man-to-bits with Toothless and let a nearly extinct dragon be slaughtered. And whenever Hiccup is in danger and Astrid is there she's the one who pushes him out of the way, endangering herself. Like with the Death Song, the Light Fury, girl was ready to sacrifice herself for the young chief.
I just think it's cool how much these two love each other they're willing to go farther than they ever would in any other occasion.